Why not a mark on the ground?I sense that it is usual, at least in RFU-land, for the primary mark to be on the 5 for the non-throwing team. At Under 15 I will mark both lines on the 5. I only give a mark for the thrower with my stationary front foot (not a mark on the ground) when I am an AR.
I personally find it very useful to have a physical mark to refer to in the event of the 'hooker' stepping off!

