My assumption would be you'd have the correct chainline if the outer ring sits in between where double rings would, so that means a BB that's shorter by the distance between the rings (half of the difference is on the other side).

But that's just an assumption... to check, measure your OLD and the distance from the centre of your cog to the dropout to find the distance your chainline should be from the centreline (OLD/2 minus cog-dropout gap). Then using your 110mm spindle, measure the distance from your chainring's centre to the edge of the BB, add 34mm, subtract the chainline figure, double what you have left, and that's how much shorter your new BB should be.