optimal altitude calculations?

hi: I have a simple question for the piloting physics majors. we all
know that planes have less air resistance to overcome at higher
altitudes, but that normally aspirated planes have less power at higher
altitudes. presumably, both are proportions of what happens at sea
level, and are hopefully not too plane dependent. That is, I would
guess that a 160hp engine would lose about the same proportion of power
as a 320hp engine. for lycomings, at 10,000', this proportion is about
50%. something similar [proportional reduction] may also happen to air
resistance, regardless of whether the plane is a cub or a lancair.

this leads me to a very simple question: on a standard day, without
any winds, what would be the optimal altitude for [cruise] speed in a
normally aspirated airplane? is this best altitude dependent on
aircraft to a first-order, or is it fairly constant across airplanes?

sincerely,

/ivo welch