Thanks everyone for the replies.
Join the dots.
Then Input a new point inside the Rwy ext (say Rwy Ext/ - 0.1)
New point is created.
Basically anytime we tried to use the RX point to make a point based off it we got the INVALID ENTRY message on the scratchpad. The best we could manage was use the RW-xx point itself and make a point 4nm back from the runway(RW-xx/-4.0).
Might I suggest though, that instead of providing him/her with a crutch to use, you allow the person to do it visually.
I really would love to do this but unfortunately this is forbidden in my company hence why I personally do not bother with visuals anymore. This also explains my associated lack of knowledge of the RX point! SOP is full use of AFDS on visuals down to MUH if desired.