Fractional marks are great as long as they're at visually significant check features. A half-way mark in the middle of a field is useless. Moving the marks a bit either side of their mathematically correct position so that they're over/next to a town, motorway junction etc has almost no effect on the maths of navigation but now means that you've got a check feature. Divide the track up into chunks of between 6 and 10 minutes by using halfs, thirds or quarters as appropriate. Use one of the normal track correction methods and have a work cycle based on the time between fixes so that you spend lots of time looking out and flying your heading accurately; leave navigating to the couple of minutes before a check and a minute or so afterwards while you do the sums and make the corrections.