If your trays are slightly bigger than you need, no big deal. Just fill them to the limits of the light coverage.
I use 500ml (ea, a & B) in my 125L tanks, for tank 1, 2 & 4. Tank 3 is for weeks 5 & 6- it gets 375ml A & B and 125ml PK13-14.
You can paint the floor if you want but it's...