With knits lately I've been "cheating." Instead of doing a regular slash/spread FBA, I've just been blending between sizes as I trace or cut, like this:
I'll start with the size that corresponds to my upper bust measurement. For this drawing, that would be the Gray size inside the black outline. Then I move out at armhole level to the size that corresponds to my full bust. Here, it's the Blue size. And then I gradually morph downward to the size that corresponds to my hips. Here, it's the Purple size. I end up with a pattern that resembles the Gray plus Red outline.
This cheating method works for me because I also usually also have to increase the bicep width in sleeves and the morphing shown above does increase the sleeve size needed. But now, instead of actually slash/spreading the sleeve, I just use a similar morph between the sleeve size that matches the Gray size I started with and I extend the underarm seams to the Blue size, like this:
As I said, I use these cheaters mostly for knits. (And before you ask, No, I don't find that the front bottom hem rides up. Your mileage may vary.) I will also sometimes use these morphs as a starting point for wovens too, because then I'll need a much smaller dart/spread when doing a traditional FBA.