I like the approach alot Ben. I personally Use 10x for the exact reasons you stated. 7.5 would be straight up, but since sometimes we'll lose, other times we'll not get paid, and oh, we do need to make a little profit, 10x barely works.

I'll be the first to admit that 10x is very thin value, but I don't usually mind high variance to max out the EV. I think 15x is safer, but you are losing out on to much possible value here. I also feel the player that waits for 15x is going to play tighter in general, which will probably inhibit slightly the chances they get paid (which may also be why they need a 15x average).

Everything in moderation... (Including moderation )

The way you calculate how much more you need to win is exactly right though. Venice and Lapidator do make so really good points that are very important to properly set mine.