+1 to legion tax
but your way is BAD way!

it should be added to ship Upkeep, for example 10% tax would mean you pay 10% of your ship upkeep (before racial and module modification).

upkeep could be modified this way :
- module upkeep : 1.000.000.000 credits
- base upkeep 10% : +100.000.000 credits
- Builder discount : -500.000.000 credits
- dark assembly bot : -50.000.000 credits
Finaly upkeep : 550.000.000 credits (100.000.000 goes to base fund)

one leader could therefore modify % upkeep to fit daily base needs as the legion grow
also there could be some new base wide buffs that would cost credits

tax can be:

bu-base upkeep
pr-player rank
troamp-total rank of all marked player(need to pay tax)(only active players)
tpndp-tax player need to pay

(bu:troamp)·pr=tpndp

(base upkeep:total rank of all marked player)·player rank=tax player need to pay