Hi
For my fixie build I am presently using a loose ball type bottom bracket. I think I have a pretty good chainline by using an axle that I had around, not sure what size it is, I think it’s 110mm.
I currently have the chainring on the inside. I want to swap it for a sealed unit anyway, and was thinking it might be nice to be able to put the ring on the outside.
How do I calculate what spindle length I need?
If you’ve currently got a BB that gives you the correct chainline with the chainring on the inside, a BB 5mm shorter will put the chainring on the outside.
What cranks and BB are they?
The cranks are Suntour ARx, and the bottom bracket spindle is a simano unit, I havent measured it, it was trrial and error with the axles available to me, though I think it to be 110mm
With a symmetric spindle you would need to go 10mm shorter (5mm per side). Or 5mm shorter from the bearing race centreline to driveside end on an asymmetric spindle.
Thanks Blakey. I should’ve said you’ll need a BB 5mm shorter on the drive side.
As I had the vernier calipers out yesterday on my cranks, I thought I’d mention that the difference between inside and outside chainring position was 7mm to the centre of the teeth.
Stop being a pain in the arse 
I just measured Shimano 600, Chorus, Record and some cheapo cranks and they’re all around 5.5mm or so. It is difficult to be accurate though.
I also measured a Centaur crank that had no rings on it and that is about 6mm between rings.
I guess they probably all vary slightly but so long as your within 2mm the chainline will be fine. The other thing is that with square taper cranks at least, you can’t get BBs in 1mm increments so you have to settle for something close.
I’ve always had good results with Shimano square taper cranks by taking 5mm off the drive side to get road cranks working with the ring on the outside.
But as they say…YMMV.