BB spindle length/chainline for re-used Shimano FC-M391 chainset I'm trying to rebuild an old Univega MTB. Primary goal is to build a good or decent bike from used parts.

The crankset+bb that was on the bike when I got it was really worn out. I unscrewed it and cleaned the frame carefully. I asked around and found a friend that had a Shimano FC-M391 crankset with its original BB I could get. 

When I installed it I just noticed that the new BB axle was ~10mm longer than the one I removed from the Univega. Unfortunately I didn't measure it but I guess it's 123mm. The original from the Univega was 110,6mm.

The new crankset doesn't come close enough to the frame. From what I can see I would like to have it ~10mm closer? I think I got it to come up all the way on the BB-axle.

I thought the BB axle length was specified with the crankset? I looked in the Shimano docs where it says 123mm.

What width should I use for this bike?
The spindle length depends on your frame and on the used crank. The combination of those two will dictate your chainline which should be 47.5mm.
In some cases it can be 47.5-50mm ( see https://www.sheldonbrown.com/chainline.html )
There are also chainlines for other types of bike, the 47.5mm are for Mountain Bikes.

You can measure your current chainline and then calculate the spindle length you need based on the current chainline and spindle lenghth. So for example if you now got a chainline of 52.5mm and need 47.5mm your BB spindle should be 10mm shorter to achieve this.