I've found that the 1 in 60 rule can often be used to good effect in these cases. So you've done your sums on the ground, you've started on the first of your navigation legs and you find out you're off track. If you can use the features on the ground to estimate how far off track you are, and how far along the track you are, it's then a fairly easy sum. If you've flown 60 miles and you're 1 mile off track, that's a 1 degree difference, so get back on track and adjust your heading by 1 degree. If you've flown 10 miles and you're 1 mile off track, the long side of the triangle is 1/6th of the original, so the angle will be 6 times as much, or a 6 degree change. If you've flown 10 miles and you're 3 miles off track, you multiply the original 1 degree by 6, and again by 3, to end up with an 18 degree change.
You can use this to figure out how to get back on track after a particular number of miles, but keep things simple for now and just try to figure out the correction angle. This method is really useful if you've got several legs that all point in the same general direction. If you've got a route planned with many large heading changes, you may not have many opportunities to use this. I always encourage students to practice some of these sums by themselves before they go flying. In practice you'll find that you generally are between 1 and 5 miles off course, and somewhere between 5 and 15 miles along your track. So practice with those numbers and after a while you will be able to quickly come up with the answer. Also, remember that a one degree difference isn't all that significant. So, if you're 10 miles along and one mile off, its 6 degrees correction, 12 miles along and one mile off, 5 degrees correction, 15 miles along and one mile off, 4 degrees correction. So just round off to those numbers and you'll be fine. Just don't forget to multiply if you're more than one mile off track.