Dropped $180 on a brass backing hammer and it fixed my rounding in one session

I bind books out of a spare room in my house in Akron and for about 8 months my spines kept coming out lopsided. Not terrible, just a little off when you laid them flat. I was using a cheap tack hammer from a big box store, maybe $12, and the head was too small so I was hitting the same spot over and over and denting the signatures. A guy at a bindery meetup in Cleveland told me the head weight matters more than how hard you swing, which I did not believe at first. So I put down $180 on a proper brass backing hammer with a rounded face and the difference was night and day. The weight does the work, I barely have to push, and my last 6 books all round evenly on the first pass with no touch ups. Cost stung at the time but I have not dented a signature since. For anyone who rounds and backs by hand, what weight are you using on your hammer?