On the back, the rail is directly below the dog holes, on the front the holdfast will bump the rail slightly, and in each instance, figuring a 3/4 inch board under the holdfast, 1 1/2 inch shorter will do nicely, but 1 3/4 inch would give you some slack for thinner stock.
My practice is to put a piece of scrap 2X3 on edge between the hold fast and the stock being held. This elevates the hold fast a little an makes it an easier target when hitting it with a mallet to remove.
jim
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)