Having a problem figuring out values of actions on a hand.

[Daliman]

Let's say I am in a hand where I am in position on the river with an 85% chance I have the best hand. When I bet and win, I make $10, When I bet and am losing, I lose $20 though, as I am check-raised. It's pretty easy to figure out what the value is of betting this every time;

.85 x $20 = $17
.15 x $40 = $6
17 - 6 = $11

So each bet is worth $11 if bet 100% of the time, (right?).

But what if I bet 85% of the time randomly? Or 90%? Or 75% How do I figure for these. I made a few attempts at it, and the #'s I come up with just don't look right at all. I'm trying to figure mainly for this problem at 75, 85, 90, and 95%. ANy help would be greatly appreciated.