Each center/region at freight is a little different, mostly due to how sharp a safety man you have. Bottom line at freight is: If the computer says it will take you 4 hours to run each leg of your run (computer takes into account each state's speed limit), and you log it that you did it in 3.5 hours, you were speeding. Safety guy may pull you aside and quiz you as to your heavy foot, or he may just hop in his car (because heaven knows he has plenty of "do nothing" time), follow you on your run, and bust you on the spot.
I know home time is important to everyone, but why jeopardize a 75-100 thousand a year job, for an extra half an hour at home. Use that half hour at a rest stop and take a bathroom break, and you won't have to do that at home, thus getting that half hour back. LOL