If you are using a single diffuser, I would start at +/- 10 per second and measure every 30 minutes and adjust accordingly. I would guess this may be on the low side, and you will need to move the water quite a bit to get the CO2
to circulate through the entire tank. Obviously, you will want to take measurements from various areas AND depths to gage appropriately.
If possible, you may want to try two diffusers, one on each side of the tank.
I have one diffuser on my 75, and I sometimes have issues getting the CO2
to all areas in my (heavily planted) tank, even with two powerheads. My plants all pearl like crazy though! Good Luck!