Simple hand, CO opens 3bb, BT calls, flop comes: 9d 7c 2h and pot is 7.5bb. CO bets 5bb.
BT needs to defend a range that stops CO profiting by betting ATC. Using GTO math this would be 60% of his hands, but i want BT to defend enough hands so that CO can't cbet 100% of hands since they all have some kind of equity to freeroll him with.
So i want to make CO breakeven when betting a hand that has 10% equity for one street(since he is OOP and BT can bet turn). I found 10% to be about accurate for hands under his optimal cbet range.

Im not sure if the calc. im doing to workout CO's EV on a cbet is correct though:
CO's risk:
(total pot when money goes in on flop=21.5bb) * (CO's equity: 0.1) = 2.15bb
5b - 2.15bb = 2.85bb

Then do the following calculation altering the % BT folds/calls till i get the answer as 0:

EV cbet= .4 * 7.5 - .6 * 2.85 = 1.29bb
EV cbet= .3 * 7.5 - .7 * 2.85 = .26bb
EV cbet= .28 * 7.5 - .72 * 2.85 = .05bb, so BT needs to defend 72% of hands?
Have i done this right please?