When one climbs in the Baron (in this case), you will notice that one has to keep increasing the throttle setting in order to maintain the Manifold Air Pressure. Eventually, you will need full throttle to maintain the required manifold air pressure and any further climb will cause a reduction.
In the real world, provided that less than 75% power is set, the mixture can be leaned for best economy.
So.......the best altitude for a particular power setting occurs where that power setting is achieved with the throttle full open.
If you can get your hands on a manual (ask the manufacturer nicely), it will most likely have settings for 55, 65 and 75% power.
The maximum speed and maximum range for a particular power setting occurs at the altitude where full throttle is reached.
Taking the Seneca as a guide since I have the fugures;
75% Power requires full throttle at 5000ft
65% Power requires full throttle at 9000ft
55% Power requires full throttle at 14,000ft
All the above with 2400 RPM
Or to look at it from a different angle......if one starts a climb with full throttle and 2400 RPM, the engine power will be:
100% at Sea Level
75% at 5000ft
65% at 9000ft
55% at 14,000ft and
45% at 18,000ft..............assuming standard atmosphere!
You can see from the above that using 2400RPM, it is impossible to get more than 75% power out of the engine above 5000ft. That is why manufactures allow leaning in the climb when above 5000ft......you can't damage the engine.
Finally, note that the mixture setting for best economy is leaner than the setting for best power (best speed). Thus, one can not always achieve best range and best speed at the same time.
Regards,
DFC