Update: The first STL is for just spacing off for a cut with the piece under the straight edge being the part being cut to length, but then had need for the part being cut off to be the correct length so made a simpler gage that you can use in either direction.
Gage to set the straight edge to do a hand drag cut with a Hypertherm Powermax 30XP hand torch with a general purpose drag tip. Offset is 3.81mm and this little tool allows me to draw a line precisely where I want the edge of the plate. Set the inside edge of the gage on that line then snug up and secure the straight edge to the outside edge of the tool and get the cut right up to the line.
Link to Fusion 360 model. https://a360.co/2HG2MXM