I just finished my checkride 2 days ago and what worked for me was a simple watch w/stopwatch. I recorded the actual time when we took off, and started my stopwatch to record TOC. I just reset it to zero after each check point and that way with a quick glance and no thinking involved I had my time when I reached my next check point. When we did the diversion, I wrote down the actual time I diverted (turned) and reset my stop watch to zero again. When I got my new GS and spun the E6B to get my time to get to my diversion airport, I was able easily say "it'll take 12 minutes from when we diverted and (I took a look at my stopwatch) we've already gone 2 minutes, so it's 10 minutes out and we'll be there at ~9:45am, that'll take about 1 gallon of fuel to get there (rule of thumb for the 172 we were in was 0.1g per 1 minute of flight)".
Good luck with your Checkride!