Was your engine delivering the same HP at FL200?
Well, I think so. I was running at the same MP/RPM as at sea level. I normally cruise at 25"/2200 RPM (76% according to the book). My plane is only turbo normalized, the turbo can still provide 25" at FL200 which is the official ceiling, and still has some oomph left though I've never been higher than that. I'd guess you could keep 25" to about FL230.