Figure out the max you want in the whole tank.
1ml per gallon is pretty safe.
Then squirt that much directly into the algae into as many spots as you can.
If you want to drain the tank to the point that the algae is above the water you can add more H2O2, up to 2 ml per gallon, but you must wait for it to quit bubbling before you refill. Few minutes, maybe 15 minutes. This higher dose is mostly broken down before you refill, which is why it is OK.
If you have more algae than you can cover with one dose then wait 24 hours and do a water change before dosing. We know the H2O2 is broken down, but on the 'better safe than sorry' side, a water change will further dilute whatever H2O2 remains in the tank.