You will find that the generally accepted method of SP to Betfair SP conversion is to increase by a set percentage based on an odds range. For instance, increase by 10% for all odds over 5 and less than 7. However this is not the most accurate method.
In TSM we use several quadratic equations to estimate the Betfair SP. Our research has shown this to be over 3 times as accurate. The formulas used take into account over 4000 horse racing tipster selections as recorded in racing-index. Every 6 months we re-check our formulas to see if they are still accurate and adjust them if necessary.
This feature is available in both Versions of TSM. Personally I think it is one of our best kept secrets !