simple math problem

[Harthor]

hey guys, i'm a math donk and i've been trying to figure out a really simple problem but i can't...so if you can help me i'd appreciate it.

HU 100bb we open xx for 2bb on the button, villan is 3betting 20% flatting 55% and folding 25%, we are planing to cbet 70% of flops for 2,5bb-4bb and is going to work out 50%, how do we figure out the EV of the play?

0.25% we win 1.5bb
0.20% we lose 1.5bb
0.165% we give up on the flop and lose 1.5bb
38.5% we cbet 2.5bb-4bb getting 50% folds (this one is my problem because it feels wrong to do:
.50 * 4 - .50 * 2.5 given that we are adding what we bet preflop to our reward)

The example is very likely -EV and not very important but i want to know how to do this calc so i can apply it in other spots