My books, from when I home raised them in the late 70s early 80s pegged it at 4:1, but that may have improved with newer meat breeds.
OTOH, the fat, bone, and water-content ratios are much better per pound than for chicken.
I should add, that I never wanted for fishing worms, nor did I ever have better gardens, than when I raised rabbits.
Good luck with your rabbit farm. It sounds interesting.