Pack manufacturers' websites have specifications for each of their packs which include the maximum recommended weight. Look that up and subtract 5 lbs. However, pack fit is a very individual matter, so it's better for you to test this number yourself.

As Rick suggests, before doing anything else, load your present pack back up with the added gear and try a hike or two, even if it's just a couple of hours around your neighborhood. Be sure to include food (or the weight/volume equivalent) for the longest period you'll be out, and a day's water (water is heavy!(. See if the pack remains comfortable for you! Remember that the reviewers aren't you!

When shopping for a pack, it's best to pack up your gear and take it to the store. Gear, food and water carry far differently than sandbags.

The most important aspects of a pack (I say this every time!) are fit, fit and fit. . The pack must fit you, must fit your gear and must fit and be comfortable for you to wear with your gear inside. Pack fit is almost as individual as shoe fit. For example, I can't wear Osprey packs at all--their hipbelts cause me considerable pain. Lots of people love Osprey.

As we often say here, Your Mileage May Vary!