Typically, pipe centres for your average towel rail are 40-45mm narrower than the overall width of the rail, so, less 20-22.5mm each side.
Personally, I'd modify pipework to go out left and right, to a 600mm one and up using angled valves - you'll get a bigger rail and the tails will look neat... Especially if you get chrome minibore
Personally, I'd modify pipework to go out left and right, to a 600mm one and up using angled valves - you'll get a bigger rail and the tails will look neat... Especially if you get chrome minibore