Generally I have people start with 2" less from English to Western. However, each style of saddle is going to fit your body differently so this really isn’t an accurate gauge. The best thing you can do is go to a saddle shop and sit in as many different types as possible. This will help you determine what style seat fits your anatomy best. Seat length, cantle height, twist on the bars, and width of the seat will all have a major impact on how the saddle fits you. Not to mention you should try both padded and hard seat saddles.
Personally I prefer a flatter seat with very high back and deep pocket, so ranch saddles are ideal for me, most barrel saddles work as well. I also hate padding of any sort, it makes me sore after a while, my saddles are all hard seat. Cutting saddles are typically very flat, but the twist is too narrow for me and the pocket isn’t as deep as I’d like. Roping saddles can go either way depending on their build, some work, some don’t. Reining saddles almost always have too much rise for me. The height of the cantle will also effect seat length. The higher the back, the longer the seat you’ll need otherwise it will push you forward rather than sit you down. I ride a wade with a 4" cantle that has a 16" seat and that’s perfect. I’ve also got a roper that fits well, has a 2.5" cantle, and is comfortable at 15". My other saddle is a barrel saddle with a 3" cantle that works for me at a 15.5" seat. I spend a lot of hours in all 3 of these every week and they all fit me well. Go sit in as many as you can. It’ll do you much better than guessing.