Concert Band Panning/Mixing

I, also, do this for myself for fun. But that doesn't mean we don't want the best result. And there's the rub. What is the best result? Recording studios have set standards they use for their productions. What they produce is what we are used to hearing as far as recordings go. But is the standard accurate. Recordings are how most of us listen to music. But then we go to a live concert. What we hear is different. And part of why it is different (along with dozens of other factors) is based on where we are in relation to the group playing. Recordings place us front and center. Great. But is that realistic?
More than that, just listen to the oboe player in the live concert. The player blends their sound just so, based on what they are hearing around them. The beautiful solo is totally dependant on what else is happening around them. With a few thousand dollars worth of samples and a DAW, you might be able to approach this. At least enough to convince most listeners. There is nothing like live music.
On the other hand, there is nothing like the music we are producing in MuseScore. I have been a member of a form of composers. The members that worked in a DAW often pointed out how unrealistic notation software output was. They are right. But what is realistic, and how do we achieve it? There have been those that think that adding delays or pitch variations are the way to do it. After all, real players don't play perfectly all the time. Maybe so, but introducing "mistakes" doesn't seem realistic to me. The goal of musicians is to play with, and in, and around all the other players. It is the job of musicians to blend with each other no matter what degree it is that they are placed at in the group.
I think that the job that we have as composers is to write good music. After all, a Bach fugue sounds good no matter what instruments are playing it.

What is max and min there? (Fahrenheit and Celsius)

The 150 come from New_max - New_min = 75 - (-75) = 75+75, you asked for that example. Remember your math exercises :-). It is simply the total range width.

The old equation works only for mapping 0...127 to -100...+100. It is a special case for these two ranges.

If you want to map e.g. to a new range like -75 to 75, you need to adapt this factor 1.575 and the source must also start with zero. If you have an old range not going to 127, you must adapt the value 63.5 too, this is the midpoint of the old range.
My second equation works for all mapping variants, from any range to any other range.