Not guesswork as such, more that the length of the spring is a function of the material, thickness, coil density and heat treatment.
In theory, two M110 springs can be completely different lengths but still have the same result in the gun. One of the big differences is that OEM springs tend to be cheaper, linear and not so likely to last for long whereas aftermarket ones are often made from better materials, have progressive windings etc (so cost a bit more). Quite often in my experience at least, the OEM springs are thicker, linear and made of some kind of weird alloy of steel and brie that loses its strength quite quickly but is often shorter because they're made from thicker metal than an equivalent aftermarket one made of finest Japanese tool steel.
Unless your new spring is SOOOOO much longer that you're effectively preloading it too much by compressing it into your gun, I really wouldn't sweat it. None of the manufacturers seem to be able to actually make springs that are consistent between each other, even at the same ratings. Find one that works for you and stick with it.
I was unaware that there are specific AK springs. Does that mean I need to buy a new AK every time I want to change the spring in an existing one? That would lead to me either having a lot of springless AKs or buying an infinite number of AKs.