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I tend to start out with around a 900ppm, as the water lower's it start's to go up, and it seems like the ppm goes up faster then the water is going down.
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The PPM rising as the water level goes down is normal (it becomes more concentrated/stronger), and wont necessarily be proportional either (example: 50% less water/50% stronger
nutrient solution). Their are a lot of environmental conditions that affect how plants uptake
nutrients, and that will affect how much nutrients are actually left in the nutrient solution, thus the actual PPM/TDS/EC remaining of the water. Try keeping the water level the same, replacing what the plants use daily to the original level. That's the only way you can really compare. Think of it like mixing a glass of powdered drink mix. If you take a measurement of how strong a full glass of lemonade is. Then using the same amount of mix, but less water, it will be much stronger lemonade. The only way to compare the two measurements is to have the water volume the same.
Keep this in mind as well. Whatever the PPM/TDS/EC is of the water you use to replace the used water with, that will also be added to the mix too (even if using plain water). But on the same note, if you don't do a good job flushing the system and getting as much of the residual mineral salts out of the
growing medium as you can, or pockets of nutrient solution (like often at the bottom of buckets), they will also affect the strength (PPM/TDS/EC) of your new nutrient solution as it's pumped through the system.
P.S.
I don't know what conversion rate your PPM meter is using, but 900 PPM seems low. The
Vegetables PPM chart I have recommends 1400-3500. However there is no way of knowing what conversion rate the meter used for the recommendations uses either. That's the fundamental problem with using PPM or TDS values. But here's something to keep in mind also, nutrients are absorbed by the roots through osmosis (from a place of higher concentration, to a place of lower concentration). If the concentration of elements is higher in the roots than in the nutrient solution, the elements can actually move from the roots back into the water.