The beauty of co-riders. If we are within clicks of each other, we are good. And, our adjustment is in the last mile, if needed. Lots of pre-ride measurements with different software. Eliminate all foreseeable bugs before the ride.
Having done 70 courses in the last two years, I am gaining confidence in my processes. On shorter courses, I do a preliminary mark on my first ride, which is when I do my elevation profile, then do my nails on my second ride.