The standard "norm" for HO is 2" on center, the NMRA states 12' on center for narrow gauge which works out to 1.65". As near as I can tell from the 1919 valuation maps for the RGS it looks like they used a 10' on center which would be 1.37" I went with 1.75 as a compromise between looks and being able to get fat fingers in there when the need arises. I don't have any curved parallel track on my track plan so don't have to worry about side swiping. If I were you, I'd draw a couple of curves, tack down some flex track and check the clearance you need with your longest equipment.
Scott