If you do your full print by resetting the timer for the full time, it's possible your timer isn't consistent. Depending on what type of timer you use (and I've tested mine and am fairly sure I can get away with adding up the shorter times), if you do a test strip with multiple hits of 5 seconds each, then you should use the number of hits with the timer still set at 5 seconds - not resetting the timer for 10 seconds.
I know Thomas' method sounds tougher to imagine, but many people swear by it (and it does make sense after using it). I do my test strips the way you explained you were, btw, and I don't use Thomas's method (though I've considered switching).