I have the Leigh dovetail jig and I practiced on some scrap and the joint came out perfect the first time. I'm cutting 1/2" long tails and the sockets on the pin board are 1/2" deep (I don't move the bit so that makes sense). The only adjustment I could see that you can make on the template is to move it forward or back which only changes the depth of the pin sockets, not the width of the sockets (or tails).
My problem is when I went to put the tails in the sockets they were too wide and ended up getting crushed (pine) instead of sliding in like the scrap test piece did.
Another thing worth noting is that I changed the finger layout for the final piece (it was wider) but did not change it between cutting the tails and the pins.