If youre using a jig... practice will get you there... filing to the jig is important in many cases... but where you say its too long, Im guessing youre free handing it... and about all that you can do is measure and draw it out as best you can and stay inside the lines and then file it larger to the fit you want...
Like most things you want to fit together, you cut under where you want to be, and then work out slowly to size - test fitting along the way.
Wade