There is nothing you can use as a rule of thumb.
As CJ has pointed out, some 7.5T races at GP are actually 1m16.
The game is filled so many errors and variation, you can't make any assumptions.
MY solution has been to stop play about 99% of all turf races anywhere.
I only play if I have TF data for the races, because CJ goes that extra mile and looks into his figures. Those who use the data from Equibase are kidding themselves - EB is a joke. And so are most all of the tracks. Look at the top of the heap -- SARATOGA!
Can't even put the gate in the right place! This is the mark of a slip-shod outfit. NO excuses for this gross incompetence.