If you use the bag rate, you will get 0.72 lb/k N and 0.36 lb/k K. It will take you 4 applications to get to the 3 lb of N. Those 4 applications will get you 1.5 lb/k K. You can put safely put down (as far as NPK, not sure about how much is safe regarding the herbicide) 5.5 lb/k in one application. You could do three applications at 5.5 lb/k to get 3 lb/k N and 1.5 lb/k K. The bigger problem is you will be putting down herbicide unnecessarily. Maybe you could use some now but you are using this to make multiple applications and at those other times you could be stressing your grass and soil life unnecessarily (any herbicide use is at least some stress). Can you take it back and get something without the herbicide? If you have weeds, it's better to use something that is just herbicide. For fertilizer, it's better to use something that is just fertilizer. If you can find something with a higher K number, that would be helpful.

Now to your specific question, how do I know much to put in the hopper? You weigh it. If you don't have other scales, you can get an empty bucket, step on your bathroom scales holding the bucket, then start filling it until you get to the weight needed. Then put it in the hopper. Set the dial on your spreader for a low number that still allows product to come out and then make multiple passes back and forth. Go east west, then north south, then switch. Keep going until you run out.

The "weed" part is a problem, because it will have a maximum dose for the herbicide that may clash with the fertilizing requirements. I also thing that taking it back for an herbicide free fertilizer is a good idea.
CALCULATIONS:

A bag marked 16-4-8 contains 16% nitrogen, 4% phosphorous and 8%
potassium. To determine how much of each is in the bag, multiply the
percentage by the weight of the bag.

Example: For a 50-pound bag:

.16 x 50 = 8
.04 x 50 = 2
.08 x 50 = 4

The bag contains 8 pounds of nitrogen, 2 pounds of phosphorus and 4 pounds of potassium.

******************

You have 50 pounds of 18-0-9 fertilizer

.18 x 50 = 9 pounds of nitrogen
0 x 50 = 0 phosphorus
.09 x 50 = 4.5 pounds of potassium

My soil analysis revealed I needed 3lbs of N and 3lbs of K per 1,000
square feet (obviously spread out over 3 or 4 applications).

Your lawn is close enough to 3000 square feet to call it that ... you need to apply 9lb of N and 9 of K over the summer. (3 feedings?)

If you spread the whole bag, evenly, over the lawn you will deliver the 3 lbs of N you need (3000 square feet @ 3 lbs per 1000 = 9 pounds) and you will come up short by 4.5 lb on the K.

You can buy "potash", which is a 0-0-X fertilizer and apply it separately to make up the difference, or mix it into the other fertilizer. It will take very little because it's usually 50-60% potassium.

At 50% K, you only need 8-10 pounds of it.

Thank you both for helping me - while I took the time to figure out the calculations, I still wanted some reassurance before hitting the lawn. I wasn't sure that if I needed say only 10lbs from a 50lb bag would it be silly to "weigh" it. I now know the answer is go ahead!

Before Becky responded I actually freaked myself out and just returned the bag to the store. I've always gone organic and went to my mainstay, Milorganite, for my first treatment. My lawn is really just deficient in potash so I may just end up applying that separately next feeding. But now I know how to calculate accurately! Thanks to all.

Any recommendations for a potash treatment???

Sulfate of potassium (SOP) is 0-0-50 fertilizer. It is the best choice (no chorine). Muriate of potassium (MOP) is 0-0-60. If you are low in magnesium, langbeinite (sold as KMag or Sul-Po-Mag) could be used. It has 22% potassium, 22% sulfur, and 11% magnesium and no nitrogen or phosphorus. If you are high in magnesium, use something else. I looked online for "potassium sulfate granular" and found some small bags at Kelp4less, 7 Springs Farm, and Walmart. Shipping costs almost as much as the product. Maybe you could get Walmart to ship to the store and save on shipping. I don't know. There is powdered SOP out there. You need the granular product. Maybe your local garden store will have SOP. Or maybe they will order it for you. Be careful about nitrogen on fescue in the summer. Most nitrogen should be applied in the fall. Don't use any in the summer. The SOP is okay in summer as long as the grass is not stressed. Otherwise wait for fall. Water it in.