Yep, if it's too far back it makes the rotation at liftoff more harsh. And when you roll on during landings if it's too far back it can force the nose down more harshly.

There's no real set number. But the 10% back sounds like a darn fine number. If it's something close to that you should be fine. But it's also nice if the model doesn't sit down on the tail if something tips it up. So some slight fudging might be needed. With wire just bend it. If you use a sheet metal gear or a similar style of made up gear you can alter the wheel positions by shimming the front or back of the sheet metal mount on the fuselage.