I am not sure I understand exactly what you are asking but the undercabinet lighting now is a tiny strip barley visible but I need to see the cabinets to understand the 2 choices and where they go

I too am confused by your questions.

When you say valance, do you mean light rail molding, like what is pictured at the bottom of the cabinet in the photo?

The higher you hang your upper cabinets the more you are going to see the underside or the cabinet even with the light rail molding in place. Someone on this forum often recommends that upper cabinets be installed high like 21" above the countertop. I often wondered how that is done with under-cabinet lighting and how that would look if the under-cabinet area is not attractively finished. My unfinished under-cabinet area is not attractive. I can see it when I am sitting at the kitchen table. It used to bother me but now it doesn't any more. In my case, I don't have a mix of uppers that are finished in white and some that are wood grain. I think it would bother me, and like you, I might entertain painting it.

Under-cabinet lighting should be installed near the front of the cabinet. I installed mine a little bit further back from the front for reasons that were important to me (I'm short and didn't want a light rail raising the height of the uppers, so it was necessary to sit the under-cabinet lighting a little bit back from the front of the cabinet to conceal it from view. If you install under-cabinet lighting too far back, that lighting will be less functional to your work surface and will illuminate your BS, which could amplify tile lippage or distort the color of your BS, which could be unattractive.

There are two ways I have found to avoid hitting cabinet doors. One is to get narrower cabinet boxes: if you have a 20" box with two doors, then each door is about 10". Since the upper cabinets are 12" deep and the countertop is 25" deep, the door doesn't stick out beyond the countertop.

The second way is to get inset cabinets with butterfly hinges, so the doors fold flat against the adjacent cabinet. European style hinges don't allow 180 degree opening; I would never have those again.

In my opinion, these two choices together are better than setting the upper cabinets 60" high.

Thanks everyone. I guess I need to install the cabinets first to determine how to position the lighting. I can research cove lighting to help design a valance shape that masks the glare while providing enough light.

Here are photos of the tops of each cabinet (painted cabinet is on right). I assume the bottoms are similarly painted, IIRC. I’ll probably come up with a plan to hide all the unfinished edges and corners, as well as paint (or clad or veneer) the bottom white, since there are only 4 cabinets. Even the painted cabinets don’t look finished on top (or bottom). There are some exposed staples behind the front frame on some cabinets - it would be nice to make all the bases look finished. Since I’ll be doing this myself, expense is not an issue.

Another way to avoid hitting upper cabinet doors is not to have them (for example, use a pantry and hang pots and pans on walls), or to have cabinets with sliding doors. Both have drawbacks.

My Mom’s dream kitchen had no upper cabinets. She insisted. She had a full pantry wall on one side and a super long countertop on the other, facing her view of the water.

We could probably learn a lot from commercial kitchens.

@theresa21: "I too am confused by your questions."

Yup.

"I guess I need to install the cabinets first to determine how to position the lighting."

No, there is no guesswork here. Correct placement of undercabinet lighting is toward the front of the upper cabinets, just behind the light rail. That way, the light is shed onto the center of the counter beneath it, where it is most useful. If you install the undercabinet lighting toward the back, you are lighting mostly the backsplash where the light is not needed. That's a lot of money to spend on undercabinet lighting to light the backsplash. Furthermore, when you place the undercabinet lighting just behind the light rail (which you are calling a "valance", but it should be a light rail), you will not see it from a seated position.

I do not know why you are installing new LED undercabinet lighting in that big bulky housing. One of the major advantages of LED undercabinet lighting is how low-profile and slim it is. I do like a diffuser on LED undercabinet lighting, but you need to shop for better quality than what you are showing. Better quality meaning a modern slim-profile LED undercabinet light.

"The entire kitchen will have close to 4000K light."

What does "close to 4000K” mean? Is it 4000K or not? In any case, an entire kitchen lit by 4000K light is a surgery operating room, not a kitchen. 4000K is fine for your undercabinet lighting, but your overhead lighting should be a more comfortable 2700K or 3000K.

"Any suggestions as to how that valance should be designed and sized are welcome."

Typically a cabinet manufacturer or woodworker provides a light rail (it's not a valance) that goes with the design of the cabinets. I.e. they can be designed with different profiles, such as just plain flat, or in Shaker style, or more ornate with beading and other embellishments that match the cabinetry. Light rails range in thickness, depending on the design of the cabinets, typically 3/4" to 1-1/2" but there are other thicknesses too.

"I know this is not ideal, but I ALWAYS hit my head on upper cabinet doors and they won’t be used much anyway."

I do not believe this, sorry. When you open a closet door, does it hit your body? No, you step back. When you open a base cabinet door, does that door hit your legs? No, you step back. When you open an upper cabinet door near to your head, do you just stand there and whack yourself in the head? How is that possible? Look, if you don't want upper cabinets, don't get them, lots of people don't get upper kitchen cabinets. It's nothing rare or innovative.

Here is the underside of my upper cabinets, you can see the plugmold in the back and LED lights (3500K) in the front. No need to paint the underside as my cabinet maker installed a finished thin panel, in the same color as the cabinets, to cover the area once the outlets and lights were installed.