We have almost the same tanks. I have 240 watts over my 90.
On waterchange (60%) I add 15 to 20 drops out the enema to water that already has PO4 from the tap, to the tune of about 1ppm. So I am guessing I have it to about your "overdose" level frequently, to no ill effects at all.
90 Gallon, eco, pressurized, controlled, 240 watts.