[split] Distances between files and ranks

[Nathan Ross]

if the  legionaries were to be replaced / rotated through the 8 rows they would have to been double spaced

I think the idea that the lines or individual soldiers were somehow rotated backwards during battle has been pretty much discounted now (there are a couple of long discussions of it here and here). Nobody could work out how it might have operated, and there's no evidence for it anyway! The 'replacement system' probably just involved moving fresh units forward during lulls in the fighting.

at 8 deep for the infantry alone each man would have taken up 6 feet 

"legionarius frequens ordinibus", Tacitus says - which means something like 'in crowded ranks'. 'Close order' is probably what he intends here.

So I think we could assume they were shield against shield, at least while they were holding the defile, 3ft per man or so, which would give a width of 3000ft / c.900 metres at 8 ranks deep.

But it could have been more - we really don't know!