If you want to be on the safe side, assume that 1/268 figure. I realize people are split on the issue because of some statement or other by
GW, but when I did some base decoration for my models (see spoiler) I put a
40k Marine next to a
40k Knight, sculpted the Titanicus Marine to the same height on the Titanicus Knight's leg, and got an 8mm Marine out of it.
Thus I'm fairly confident in the 8mm (or 1/268 as per FrozenDwarf's post) figure. At the end of the day
GW is terrible with scale and you can never tell what they'd do exactly if they introduced infantry into Titanicus, but you can be sure that there will be Marines and I figure they will want to keep the size ratio from
40k, give or take. Thus you're probably good if your terrain works with 8mm tall Marines.