I have a few custom vanities that are completely one offs, the problem is the way they are designed and we did a lot of the design work during our initial training so they are dated by 18months or so now and will be a massive ball ache to reinvent from the ground up. 

Long Story short the width is not the width of the cabinet, its the width plus posts. When going to make them i keep forgetting and entering the purchased size, and then they are all coming out too big, luckily salvageable and can be cut down. 

My question is, with a product specific prompt, can i enter the size that it was purchased at say 1400, then with the actual width prompt like any other cabinet, have that read my prompt and minus off 160mm. My reasoning for this is so everything still points to the original width prompt. So i want my own width prompt then the original width prompt will calculate from that. I think the my theory is correct in all of that?