Iíd also check phosphates as well if nitrates are 40ppm. That would be a good indicator if you might be feeding to much if both are higher than what your ferts would normally provide.
Iíve always limited it to what they will consume in 1-2min if feeding twice a day. Slack off a bit and youíll probably see both readings pull back a bit within 1week.
Take a your prepped change water and measure nitrates before and after dosing with ferts for that volume of water. That will basically show you nitrate levels of water supply and after supplements. Any increase you see in tank levels will be from organic breakdown of plant or fish waste in tank.