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I am sick of looking in weed forums for the correct answers. Come on lets get this forum going.
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I hear ya ... I'm pretty new to hydro so I can't answer your question, but I'll listen in ... welcome to the forum!

I am currently growing lettuce and basil as my very first hydro crop ever. So far so good. I took technaflora's "recipe for success" and scaled it down to 275 PPM (my water adds about 200 to it). I am using a spreadsheet where I put in the number of gallons for the reservoir, and it automatically adjusts the amounts of each item I need to add. I have a similar setup for adjusting PH.

Here is what I use, scaled down to 25% strength:
http://www.technaflora.com/recipe.php

What I'd like to learn how to do is how to adjust the recipe to suit a PPM a little better. Right now I am taking their full strength recipe (too strong for lettuce), at 1100 PPM, dividing it by 4 to get a 25% recipe of 275 PPM. If I want exactly a 500 PPM, I'm not sure how to convert that yet.

Well normally you look at a starting ppm say 20 or so for r/o water. Then you are gonna figure out the p-k-n that is the main key which will give you your nute strength ppm. My question is how non p-k-n mixes and if I push the ppm up with these will the plant get nute burn from the total ppm that of supplements and p-k-n mix or simply the ppm of p-k-n mix. Your adjusting the recipe to a chart try an ec meter they are the only way to know whats going on in the res.

Yes, I've been checking the mix with a truncheon nute meter, and it's been right on what I have expected it to be.

It was off this last time -- 400 PPM instead of 475 - but this may be due to me not measuring the gallons of water very accurately. I'm not sure if it's ok to add more nutes now that everything is already mixed, or else I'd like to move the PPM higher for the week...

If your starting ppm is 200, you can go higher than 500 ppm (total) - somewhere between 600-700 ppm. The 200 ppm from the hard water will be mostly calcium and magnesium - which won't contribute to nute burn at that level. Too much nitrogen is usually the culprit in nute burn.

If you want to nail things down to a science a reverse osmosis system as Evan suggests would be a good investment. You can get one for less than $100 at an aquarium specialty store.