DWC: Should water level be consistent?

I am in flower cycle and the plant is drinking like crazy.
Should I keep the the water level the same? What is the
best way to do this? Mix up some water with the same nutes amounts
and add as needed?

top off with fresh water daily, nute change once a week.

adding more nutes without knowing and exact npk and what the plant has and has not consumed will lead to toxicity and/or ph imbalance. ppm and ec doesn't really help you much other than to know the plant IS taking up nutrients.

I start with 5gal in a 5gal bucket, when it gets down to about 1-1\2gal I change it with a fresh 5gal. The roots will wick up the nutes so no problems there and plants do great, they get consistent fresh res when needed, not watered down nute ratios from adding more water.