I think you should just use an adjustment factor for multitabling.

Use a number like 1.15.

So for your calculation of singletable 50nl you'll run:

50*20=1000

Two table:

50*20*1.15= 1150

Four table:

50*20*1.15*1.15*1.15= 1520

And etc. That might temper your number down a bit.