I agree with the 25% rule, though if you're 300 lbs I wouldn't plan on a 75 lb pack! I typically carry between 38 lbs (good weather) and 45 lbs (cold or wet weather) for a 2-3 day hike on Whitney. I've seen a few people who managed to get down very close to the 30 lb mark with some super-light gear, bivy sack, no spare clothes or luxuries, etc.