You can calculate ideal speaking lengths by using a simple ratio
Start at note 88 which can vary from around 49 mm to 51 or 52mm
Then multiply by 39 and then divide by 37 to get a speaking length for note 87 and so on.
But speaking lengths is only part of the picture.
To keep the other parameters so that power and inharmonicity and more relatively even the wire diameters will need to get larger and starting around note 31 to 27 the addition of copper winding is necessary. Many times the upper bass notes are shorter than lo tenor.
The shorter the piano the more these compromises need to happen.
Can’t help with averages, sorry.
I'm a bit confused. So I multiply by 39/37 to get note 87, and then I multiply 87's length by 39/37 to get note 86? So the magic number is the ratio 39/37 = 1.054? If I do that calculation 61 times (i.e. 4 octaves) that puts me at a length of about 50" for a C3 plain string. I guess that checks out?
If that is correct, can I go in reverse from the maximum length of the lowest bass string to obtain all the higher bass strings? That is, imaging an A0 of 50", I would multiply that by 37/39 = .948 over and over again to get the higher bass strings? This can't be right as that would put me at about 12" for a B2 wound string, which seems too short.