[Edukashun]

It's not exact but whilst playing I use the following.

With two cards to come you multiply your outs by 4
With only one card you multiply your outs by 2

In your example 15x4 = 60% (which is slightly wrong), if you get to see both cards.
I think the correct % is more like 55 but I've been working all night and I'm shattered so I don't wanna work it out.