I'm assuming you mean where to actually make the cut for 45 dgree return allowing for overhang and fixing.
there are more qualified ways of doing this I'm sure, but as I recently fitted my kitchen I'll tell you how I went about it.
My cornice required a 2inch overhang, the overall width of the cornice was 3 inches,
so drawing a line 2 inches in from the front edge and running the lenght of the cornice, this gives the line to use for all measurements and cuts.
When you measure use this line to place you right angle against the measured mark, this way you'll be sure the over hang remains constant.
If you're using an ogee type cornice it is more difficult, but you can make a jig that will insure the cuts are in the right place, with these types of cornice you usually measure the inside flat edge, leaving enough to fix the cornice to the cupboard tops.
This is not the easiest thing to explain without a drawing, but I hope this helps.