Indeed, one of the biggest improvements in going from cyclocomputer + cue sheet to GPS for long distance navigation, is eliminating the need for math in your head. Cue sheet says turn right at 47.18 miles, from which you have to subtract 0.62 in your head due to error, and remember the result before the turn comes. Then let's say there's another turn at 47.29, which is pretty quick so you have to decide ahead of time whether to remember 0.11 and add that upon turning, or subtract 0.62 again and remember two results, or just hope there's only one road in close proximity. Keeping in mind the 0.62 becomes 0.63 after a bit, now try that short on sleep on day two or three. Then add rain, with required efforts to keep the cue sheet dry. The best is when the cue says turn at T in the road, in which case you give the brain a break and ride to the T.

An errorless cyclocomputer would have been a game changer.

With GPS you just obey the GPS. Every turn is easy like a T. Admittedly something is lost when navigation becomes effortless, but alas sometimes GPS fails and one is forced to a backup method. Which can be fun in its own way.