I've been binding notebooks for maybe 8 months, all rounded spines since that felt safer. Last week I finally tried a flat back with a stiff board and the difference is wild, no more wobble when it stands up. The trick for me was changing how I glued the mull, I stopped swooping it on and started pressing it straight down in sections. It took 2 failed attempts before this one came out clean, but now I'm wondering if I should redo a couple of my older books. Has anyone else found that switch harder than they expected?
That bit about pressing the mull straight down in sections instead of swooping it on is the whole thing right there. Most skills work like that, where the last little change does more than the first ten tries, and you only find it after the failures teach you what to feel for. I'd say redo the older books if you still use them, since a spine that stands up straight beats one you keep propping against something.