A couple of factors have to be considered. One, the contrast range that you wish to achieve, balanced against what the PJ can produce. This combines with sharpness and ouput level. The whole equation that you can achieve is dependant on what screen material you choose. If the gain is too high.. you get too much colorshift, too much light output drop-off in the corners,and too much hotspotting. It's a tough mixture to get right. The trick is to see enough screens under enough different conditons, and to get to what you need with as little expense and hassle as possible. The learning curve can be very expensive and frustrating. It all depends on how easy you are to satisfy,and when the situation reaches your personal satisfaftion point. ie,where you feel you have 'enough'.
If you push your screen size too high with that PJ, you will need to compensate with a higher gain screen material. This will allow you to keep your contrast range up a bit, without pushing the PJ too hard. For instance, a PJ like yours should not be used with a diagonal of over approximately 84". Many people push them alot higher, like 96" or greater. In these csases it is very advisable to go to a higher gain screen, like about 1.5 or so. Once you get above a gain of 1.5 or so, the hotspotting and colorshift tend to get close to the annoyance factor point. But, anything lower brings about too much pressure on the PJ's tubes, and prematurely ages the PJ. Or, you end up with a soft and dim image. This is why I made the screen paint in the 1.5-1.6 gain area. It pushes the limit, and maintains great color accuracy. If I was to push the gain higher, appliction would become very difficult, unless you where spraying the paint. Spraying is HIGHLY recomended.