Moving a face by distance measured not perp to the face
I have a slab sloping at 8o to horizontal, with a square edge face. I want to dmmove that edge face, to extend the slab (in its own plane i.e. perp to the edge face) by 300mm, but that 300mm to be measured horizontally, not measured perp to the edge face.
UCS has xy plane horizontal.
I select the edge face>click dmmove>enter first point>SHIFT to lock snap tracking>type 300 in dynamic dimension box>ENTER
But the slab extends 300 measured in the plane of the slab/perp to the edge face, not as desired.
How can I make the 300 extension be measured horizontally? Or in fact measured in any chosen direction other than the default which is perp to the edge face?
Some other workflow altogether?