The easiest way is to just use one of the province sheets floating around but if you like pen and paper:
BE (neglecting prisoners) = 50*( 1 + MIN( Peons / ( Jobs *0.67 ) ,1 ) ) * BEMods
BE (neglecting prisoners, barren acres) = 50*( 1 + MIN( (Max Pop - Military - Wizards) / ( (Acres-Homes) *25 *0.67 ) ,1 ) ) * BEMods
BE (neglecting prisoners, barren acres) = 50*( 1 + MIN( ( (Acres*25 + Homes*10)*PopMods - Military - Wizards) / ( (Acres-Homes) *25 *0.67 ) ,1 ) ) * BEMods
BE (neglecting prisoners, barren acres, pop mods, BE mods) = 50*( 1 + MIN( ( (Acres*25 + Homes*10) * (1-Draft%) - Wizards) / ( (Acres-Homes) *25 *0.67 ) ,1 ) )
So for 50% draft, assuming 1wpa (not factored into draft)
BE = 50* ( 1 + MIN( ( (1*25 + 0*10) * (1 - .5) - 1 ) / ( (1-0) *25 *.67) ,1 ) ) = 84.4%