If its really that hard for you, take your cross country as timed then add .2 on the front end and .1 on the tail. Thats a 6 minute taxi in and 12 minute startup/runup/taxi out. If you take your time, maybe .3 on the front.

Its not rocket science here. The Feds arent gonna hunt you down over .1, but its how exact do YOU want to be.

Even using a hobbs can be innacurate. Since hobbs only ticks every 6 minutes, if someone shut down after 5, then 1 minute into your flight you already have .1. Same can happen on the tail end if it ticks, then 1 minute later you shut down. You take credit for the full .1. Can help you by 5 minutes or short change you by 5 minutes. A watch is probably the most accurate of all when it comes down to it.