As a ballpark figure, 30 lbs is entirely reasonable for the purpose you proposed, i.e. loading your pack with random stuff and seeing how far you can walk with that weight. Especially since the pack will be included in that total. If you're off by a couple of pounds, it won't make much difference to the data you'll collect.

Understand that a load of random stuff will feel somewhat different than your eventual load, in terms of weight distribution.