Tarmach
My preference is to estimate - if drifting left, then add 5 deg right and see how that works - its an iterative process, but effective in application. Trying to work it our with a whizz wheel can be distracting
Despite the bad press that GPS receives from some, it is also really helpful in this area - you will see bearing to next waypoint and track displayed - a simple subtraction gives you a number on how many degrees off you are - worth having a GPS for this alone, as well as DME substitute.
.... and before all the anti crowd leap in, it's important to monitor the GPS to make sure it hasn't frozen! - also to confirm position by visual reference - I use a VFR flight log/chart prepared in the traditional manner and a GPS and find the more triangulation available, the better!