I do not really know? But I do keep up with the math everyday on the RECAPP. I use just the little green hourly ruler and from start I go over and make a slash at 14 thus I must be finished by and keep track of my 11 hours. Include your breaks, PTIs, fuel and loads it usually comes to around 13 hours on a 600 mile run.
Scientific? NO. Inexpensive and keeps me out of trouble.
My system is simple, but I am sure others is better. I have never gotten a violation from my office, so I am doing something right.