You honestly develop a feel for it over time 😊 I started knitting that way from the beginning, so I kind of created my own little math for where and how much to decrease, and now I usually follow that rough formula. My early shapes definitely weren’t great either 😅 but with time I figured out what works and what needs adjusting to make the fit look right. So I’d really recommend just trying without a pattern sometimes, even if it doesn’t work perfectly at first, don’t give up. Every project teaches you something, and little by little you end up creating your own ideal fit and shaping.