PP
I did not reply because I was going off memory and I was not going to check my source. Since in a procrastinating frame of mind I offer the following. (just as well that I checked)
This was based purely on Melbourne Metro and only rateable races. And only on the services selections. So it may not answer your question at all and may be wrong in your case.
Their finding was to use TF if you expected the SP to be <$11. If you expected the SP to be >$11 then use the Best Tote. This lead them to issue a rule of thumb TF for single figures BT for double figures.