There is this one: https://www.giantdimensions.com/ind.....tioncalculator

That's unfortunately just a height change comparison thing, it doesn't show changes based on muscle mass

As a general rule. A 26%~ increase in height, width, and depth doubles mass. While a 100% increase in height, width, depth, multiples mass by 8. But this only holds true if all proportions remain same

Yeah. This is a muscle growth story so his mass would be changing independent of his height

It's not the same, but there is this Growth drive template that the artist oldShep used to measure the changes in size. It's not perfect, but it might be a close thing to what you are looking for
https://drive.google.com/file/d/1s4.....4YT10LFSx/view
It's a pdf with the info, and included inside is a link to the spreadsheet you can copy and use for the calculations

That's a lot of reading but on a quick glance it may be almost exactly what I need

Hmm. I'm not sure I understand what you're looking for when you say, "something I can use to guesstimate muscle mass/weight". What kind of information do you want to feed in, and what do you want to get out of such a tool?

One example of how I imagine this could work, calculating by hand:
The first result on Google says that men aged 18 to 35 tend to be about 40-44% muscle mass, by weight. So if your character weighs 200 lbs. and is 40% muscle mass to start, that's 80 lbs. of muscle to begin, and 120 lbs. of organs, fat, bones, etc.

If you wanted to increase their muscle mass by 1% x times for x likes, for instance, you could multiply 80 * 1.01^x to figure out their resulting muscle mass at the end of that stage in the sequence. Then you add that value to the 120 lbs. of everything else, which one would imagine stays constant, to find out their final weight at the end of that stage.

Trying to plan a growth drive and trying to figure out how I would like up the growth aspects so I can accurately display how much someone grew by each increment.