Once you are within 6 feet or so from the ridge, check your marks. Are the last marks on each end, the same distance from the ridge (the peak)? If they are not, you can compensate. If there is anything I can't stand when I look at a roof, it's when one end has 4 inches of exposure and by the time you get to the other end, the caps are nearly covering the whole course! This can easily be avoided. Adjust your marks once you get up within 6 feet or so. Don't wait until the last course , that will be very noticable. Do it gradually. For example, let's say that you have marked lines all the way up the roof and are now about 6' away from the ridge, or peak. You measure from your last mark to the peak and get 5 1/4". On the other end, you measure from your last mark to the peak and get 6". A difference of 3/4". I know, it's not much, but it bugs the daylight out of me! By adjusting the side that measured 6" (the long side) you can fix this problem in just 6 courses.

Here's how:
Instead of measuring 5" increments, measure 5 1/8". The 1/8" will never be noticed. Snap those lines. In just 6 courses, you've compensated for the 3/4" difference. If you are anywhere near your lines when actually laying the shingles, your roof will look straight and people will get the impression that you really know your stuff! And you don't even have to tell them who told you. ;) (but you can if you want).

Complete the other side in just the same way and then let's get ready to lay some starter strip down!