As far as I can remember, when looking for a crankset, for each chainring a number in millimetres is given by what the chainline will be affected. E.g. - for Sturmey-Archer FCT single chainring series it is +4 mm. It means that the sprocket is 4 mm to the right (outside) past the end of the spindle. Knowing this you need to find the correct spindle to get the chainline to the desired value.
I'll share my somewhat ghetto method of converting a Motobecane (thus French) frame to fixed-gear where I've put a cotterless spindle.
Since the drive-side cup was seized and French threads are not available for cups of cartridge bottom bracket I've came up with an ingenious idea to keep the existing cups.
I've managed to find a spindle (Miche Team 68x107 with BSA cups) where the outside diameter of the cartridge bearings almost fit the inside of the cups (tiny bit of filing the bearings helped).
To compensate for the wider bottom bracket of Motobecane frame and to keep the chainline more or less correct I've put a (ground-to-size) washer on the drive side.
This setup got me going for more than a thousand kilometres already. If I was to make a similar conversion again, the only thing I would change is to take pictures to clearly show what and how was done.
I hope it helps.