The official protocols are found in the Society of Automotive Engineer's (SAE) Handbook.
The mathematics runs to about 2-1/2 pages.
The allowed tolerances for the test track,actual test procedures,data recording resolution,continuous weather monitoring,and data reduction,I believe,is beyond the scope of the 'hobbyist,'
I baled out and hired CAR and DRIVER to do it for me,at the Chrysler Proving Grounds.They have a dead-flat straightaway track, which is dedicated for this sort of testing,hidden inside a forested area,protected from wind.There's no traffic,as it's a controlled area.
They have the the 4-wheel scale weighing system required.
Car and DRIVER provided test driver,the optical 5th-wheel data acquisition hardware,data storage/retrieval,and data reduction computer program,plus an onsite weather station.
To accurately test a vehicle would require the disassembly and independent measurement of every rotating component in the vehicle for polar moment of inertia.A daunting proposition.
Some years,there can be as little as 7-days/year, when the weather is agreeable enough to do the test.
You can do some things to experience 'trends',but even the automakers have abandoned coastdowns for the wind tunnel or CFD,where they can completely control the environment.
Just sayin'
