My calculations were correct, 150 equals to 75, and 130 equals to 65.
150 is the correct value if I match to 440Hz as with the hardware sound blaster at that frequency.
But in DOSBox 150 still sounds just a little louder than a SB1.0 13020B because it has no filtering on top of the FM sounds.
Using games that mix FM and PCM as reference: SB1.5, SB2.0, SBPro2, SB16 - Audio Comparison
For DOSBox to audibly match a Sound Blaster 1.0 or 2.0 (CT1320C or CT1350B respectively) I need to set to 130 (mixer FM 65).
I chose SB1/2 as reference because DOSBox filter resembles the filter on these cards, the SBPro is much too filtered compared to DOSBox and the SB16 has completely different kind of filter.
Moreover, Adlib games that don't have in-game volume controls for music/audio usually belong to the SB1.0 or 2.0 era.
I use quality studio monitors and studio headphones for critical listening, they usually have more treble than typical desktop speakers, or should I say more "correct" treble.
Behringer Truth B3030A
Audio Technica ATH-M40X
I suggest keeping it at 150 for typical desktop speakers (Creative, Logitech, etc...), ones that have tone controls on the speakers.
But lower to 130 if you have speakers with flat frequency response that go up to 20kHz, at ear level, or headphones.
Once DOSBox incorporate filtering on the FM sounds it would be much more straight forward comparison and 150 should be the default setting.
On my setup I use 130.
I should mention once again than the difference between 130 and 150 is only 1.25db, so most people will not notice the difference at all.
But vanilla DOSBox default 200 and 130 is -3.75db difference which is undoubtedly audible.
A 3.75db quieter music might make all the difference between intelligible speech vs drowned-under-music speech.