Seems very reasonable to me but I am far from a fish expert. I know puffers are very messy fish. I would probably keep checking ammonia and nitrite weekly once you get your fish all added. If you start seeing anything detectible on either you can always add more dry rock later.
I wouldn't get too hung up on this. There are so many different types of live and dry rock on the market it makes the thumb rule of 1lb per gallon almost worthless imo. The porosity of rock will make a bigger difference to the weight than if it is live or dry. It also makes a bigger impact on how much you need for filtration. As rocks become more porous they weigh less yet can harbor more bacteria. Because of this, 25 lbs of porous rock could be a much more effective filter than 50lbs of a denser rock.
