That’s how I interpreted it.
Say original board costs were $500/stall or $250/pasture for nice round numbers.
BO increased board by $40 for everyone (8% of stall board, 16% of pasture board), so now it’s $540/stall and $290/pasture.
OP is upset that the increase was the same for everyone and thinks there should have only been a $27 increase for pasture board and a $53 increase for stall board. This would have meant that everyone’s board increased by ~10.5%, and according to OP would have been the “fair” way to split the board increases.
Of course, we’ve all discussed by now that OP’s logic only works if the increase of costs was truly proportional to the original cost of board, which is probably unlikely.