I'm hoping you mean plain incline not smith machine correct? Its difficult to gauge completely, b/c some people have a tendancy to bench with their shoulders instead of pec/triceps. Are you a flared elbow bencher, or a tucked elbow bencher?
Maxes are very difficult to judge, everyone has a different workout. I'd assume you could easily max 50lbs over your 5rm, this may or may not be true for you. Have you ever maxed before?
Best advice is just try it and find out. Start with some light tripples then move on to heavy singles up to 275. If it feels light for a single, start adding 20lbs or so at a time until you can't move it. Thats how I do it at least. Just make sure you do a proper warmup and stretching, then acclimate to the heavy weight properly.