In my limited experience (4 months floating raft system) i have found PPM goes up as water evaporates. Add more water and the PPM will drop down to where you started.

You must include the tap water reading in achieving the required ppm. Tap water contains calcium, sulphate and chlorides, which are part of the nutrients. Add more nutrient to get a higher ppm and less to go lower.

However,
A ppm reading is inaccurate as the meter reads the conductance of the solution and that is not in direct relation to ppm. Manufacturers of ppm meters use an average conversion factor, and in some meters you can actually set that factor.
I converted to µS (micro Siemens) years ago when I couldn't figure out why my home made solutions did not match the desired ppm reading.
See also: pH/PPM/CF for Various Vegetables

I have a combo meter that does PH and EC and PPM. EC is the best way to measure the correct amount of solution.

Take the amount of the PPM and divide it by 700 to get the EC level. Here is a good website I use to convert PPM to EC: http://www.4hydroponics.com/convertPPM.htm.

For example, my young pepper plants are set at 450 PPM or .64 EC. If the solution is moving at all the PPM reading can fluctuate and not be accurate. So I use the EC reading.

I have several rules about the solution that I follow kind of no matter what. First off I never use fresh tap water because it contains Chlorine which will evaporate from the water in a few hours and therefore artificially raises the PH (about 1.0). I set my water out in a 5 gallon bucket with a bubble stone bubbling away for at least 6 hours before using it. Second: once I setup the solution, I only add fresh water, and the PH up or down to it. I never add more solution because you do not know which nutrients your plant used. You can kill the plant by giving it too much of one type of nutrient. Third is that I change my solution out every 7 days at maximum.

Unless I'm mistaken a given meter doesn't really use a different measuring technique to measure ppm, EC, and TDS. I'm pretty sure they just take a measurement and then convert it into whatever type of numbers you have it set for. I could be wrong on that.

Anyway, you want to measure your water, then add the ppm or whatever you want to feed your plants to that, and mix your nutrients to that level.

Simple steps:
1. Measure ppm of water.
2. Add that number to desired ppm of nutrients.
3. Add nutrients until that ppm is reached.
4. Stir well, allow solution to sit for a few hours if possible. (Aeration a big plus during this.)
5. Measure pH.
6. Adjust pH to desired level.

It's not quite as simple as saying "EC goes up as water evaporates". If that's all that's happening, that's absolutely true. But EC will go up or down with or without evaporation in a hydroponic system because there's more going on than just evaporation. If the plants are eating a lot of nutrients compared to water, the EC will go down. If the plants are a lot thirstier than hungry, EC will go up.

Here's a real basic rule of thumb:
EC goes up, pH goes down = reduce your EC levels (too much nutrients)
EC goes down, pH goes up = increase your EC levels (not enough nutrients)
EC stays the same = correct feeding levels.

It's a bit more complicated than that, but generally true. You want to feed your plants both food and water at the rate they absorb it.