I'm trying to figure out what size spindle I need for these cranks. With the stock 117 mm BB, I get a 48 mm chainline measurement for the cranks, and my wheel gives me the standard 42.5 mm chainline with a Shimano freewheel. I know it seems obvious that With these measurements, I need an 111.5 mm BB, but that's a rather odd size. I plan on getting a fixed cog for the other side of my hub, will this alter the chainline any? 111.5 is a stupid spindle length also. Should I go with a 110.5 from Benscyle, or go longer with a 113?

I'd go with a 110. A chainline that's off by a mm or two won't be noticeable at all.

If the current bb is symetric and you want to move the driveside in by 5mm you'll need a 10 mm shorter spindle,in your case a 107.

So for every mm that chainline is off, I have to subtract 2 mm from the BB spindle length?

Yes.Comparing a 118 with a 103,the spindle is 13mm shorter but the drive side end(as the nds) is only 7.5mm shorter.If the current bb happens to be asymmetric,measure the drive and non-drive side ends of the spindle and add the balance to the total and take it from there.Example: driveside 22mm,non-driveside 17mm,spindlelenght 113,add the 5mm difference so the symmetric equivalent would be 118.