Understood George, however I think we're all knit picking here! I cheekily pointed out Jonathan's build as he has done what I was (maybe still am??!) thinking of doing and putting the rails directly on the steel. The sort of deflections I'm possibly looking at are in the magnitude if 0.5mm which is one stroke of sandpaper on a wing rib to put it in context. I've built some large and small planes by cutting out round paper templates so there would already be larger errors than what I think these rails will give.

I do think doing the epoxy levelling is worth it, after all why put in the hours on the design, the hours fabricating the frame only to not let it reach its full potential? However Jonathan's "Sufficiently strong" machine (a superb machine) was built with parts made (nested out of one large sheet of Ali) on a machine using supported round rail (I'm sure you've read on different build threads Jazz's comments on what he thinks the linearity of those rails is like!) on non levelled beams ;). Is that a chicken and egg situation??

Anyway, enough fun. I'll have a look at the rails tomorrow and poke about with some feeler gauges to explore...