/3/ - 3DCG

An issue I have had with blender when trying to do precise things, for example, this is the edge of a room.
The current verts on the left are one side of the hold where I want the door, the grease pencil on the right are where I want the other side of the door hole and I want it to be exactly 900mm over.
What is the best way to acomplish this?
The ways I know so far that all seem too convoluted are;
>add a loop, its in the middle of the two verts and I have to drag it and do it by eye or do some maths by first subtracting from the total length to figure out how far left from the center point where the cut appears, then move it x amount left.
>duplicate verts on the left, move them to the right 900mm then add a loop cut on the original face and slide the loop cut verts to the left until they snap to the verts i duplicated, then merge or delete those duplicated verts

isnt there an easier way?