No one can really give you an answer on that. It would depend on how large your TV is and exactly how much overscan you are aiming for. You'll have to do the math yourself. Look at your test pattern and see how many boxes the pattern has vertically and horrizontally and use these to divide by the height and width of your screen.
If your main goal is just to straighten out the areas you see problems in, you could always just use the existing test pattern lines and just run a string across the problem area so you atleast have a good reference. It just depends on how far you want to take things. A full and total geometry and convergence can take a very long time. You have to do a full multipoint converegence on green and then use green to do red, then again for blue. You might be looking at most of an afternoon staring at a grid if you go all out.
You'll probably want to do some research on converging Mitsubishis before you do it to make sure you don't do all that work and somehow lose it. One of the best sources for technical info on Mitsubishi TVs is www.***************.com
. Go to their forums and you'll find the Mitsubishi section. I haven't spent much time there in a couple years, but they used to have quite a few Mitsubishi experts around there if you have any questions.