In reply to iamaclimber:
A combination of things. I know I'm 55 paces to 100m with a light pack on good ground. I count out sets of 100m at a time, and at the end of each one I take a few more paces to account to the terrain over that 100m and my load, tiredness etc. I guess roughly five extra paces for a big pack, being tired, or moderate heather, and I'd take 5 off on a gentle downhill, but there's no strict rule. I add each one in turn, literally counting 53...54...55...pack1...pack2...tired1...done, 100m.
More to the point, you have to recognise the limitation of pacing. Am I actually accurate to one pace after adding the three I added there? Of course not. Don't rely on precision, and accept that every leg will be a little off. Keep legs short, so a 10% error is 30m not 100m. Pick your route so that you can correct your position every few legs, and make sure an error of 10% in your distance measurement (or 5 degrees on your bearing for that matter) won't lead you off a cliff. Sometimes, you might also encounter terrain like bog or boulders or very steep ground where you have to accept that you can't use pacing, and you have to fall back on a different technique.
If you fancy some practice, have a go at orienteering. On the harder routes, being extra accurate can help a lot with finding the controls quickly.
Post edited at 19:28