I just use:

for turn & river - outs x 4 + 1 = percentage chance of hitting
for river - outs x 2 + 1 = percentage chance of hitting.

I actually checked the above agains tthe proper method and they are pretty close. Some are a bit over, 1-2 % max, some are a bit under by about the same, but when it all comes down to it, it's easy to work out and quick to do too.

J