If you call Dynon Tech Support they can explain HP % calculations the Skyview uses. I believe it compares manifold pressure, RPM and density altitude to determine HP %. The OAT allows the unit to determine density altitude so it will need to work. You do not calculate HP. You set this value for your engine in the the aircraft parameters of the Setup menu.
Dynon published the article below back in 2014 describing how they calculate Percent Power. Dynon has tweaked the algorithms somewhat since then, but it may give you some insight as to how it's done and the parameters used. Note that the calculation is different between ROP and LOP operations.