Is there an easy way of working out what length square-tapered BB I need to buy if I'm simply replacing a hollowtech BB with a square tapered one???

It's 73mm English threaded BB housing (for my Cotic 'Soul').

Any ideas....?

I usually find 113mm works in just about all cases.

but.....it's for a pair of old Middleburn RS2's which have straight arms and so need to be spaced further out no?

but.....it's for a pair of old Middleburn RS2's which have straight arms and so need to be spaced further out no?

The chainrings should still be in the same place as they would be with 'bent' cranks. However, I have known Middleburns foul on the BB with 113mm (not much clearance between BB and lock-ring), so if you think there might be clearance issues go 118mm ... the chainline over a 3x9 varies so much that in the real world it won't make much difference.

Just give Middleburn a quick telephone to confirm. They're an obliging bunch.