What I mean by this is:
If I have a BarPeriod 120 strategy tested across 3 years (156 weeks) of historical data --
and I want to compare it with a BarPeriod 240 strategy... then I may potentially want to test it across 6 years (312 weeks) of historical data for consistency. It would seem inconsistent to compare the BarPeriod 240 strategy against only 3 years of data.
I forgot to add that I completely agree with the above reasoning. If we can agree that 10 years of data should be a minimum for say 4hr bars, than it should also be true that if testing say 15min strategy, backtesting on about 7.5 months should be equivalent.
Edit: and after a quick look, your formula makes sense, too
