Secret formula for Garmin .fit file left_right_balance message

So, been playing with Powertap (Saris) new P1 pedals. They measure power separately from left and right, so new possibilities for more data like left-right power balance. But, how is that data included in .tcx and .fit activity files?

Got Ethan's Garmin files from his Quark power meter and find values for left_right_balance like 169, 159, 148, etc. So what do these numbers mean. I would have expected to see something like 50, 48, 55. Maybe that would be the left pedal power percent, since the right power percent would be 100 minus left power percent. Stumped. The .tcx files don't have data for left_right_balance at all.

An Internet search turned up nothing about Garmin's secret formula. The thisisant.com forum on .fit files had this suggestion: " I can only recommend to take some activity .FIT files from Garmin devices and use these as examples. Both for parsing .FIT files and for creating .FIT files.
There's no 110% comprehensive spec. of the different .FIT file types (semantics, spec. of what's mandatory), so taking Garmin's files as a de facto standard should assure a good level of compatibility. "

(Later I did find the answer on the .fit forum )

But I borrowed a Garmin and took some data with various percent power on left and right pedals and found the formula to be left_right_balance = right% + 128.