You know, when I started sewing, I had no idea that such a thing even existed. I assumed (obviously incorrectly) that if you chose a pattern based on your measurements, you would end up with a garment that fits. Which was such an incredibly naive assumption, and I always thought that it was something I did wrong that made it so that I had so many fails to start with. Turns out it was something I did wrong–blindly trusting the pattern companies to know what would fit me best.
Most pattern companies draft for a B cup, but seeing as I’m an E cup (that’s DDD in American brands), there was a pretty significant discrepancy, especially when you figure in my narrow shoulders and petite frame.
The hardest part of an FBA is finding out how much you need to add and how. Lots of FBA tutorials out there simply say to add however much you need to the pattern, but they don’t tell you how to determine how much you need to add. I really like the SewLA method of FBA for the slash/spread method (which tells you how much to add), but I also like the pivot/slide method too. But how do you know which one to choose? Sometimes a pattern piece is too complicated for the slash/spread method, but sometimes the pivot/slide method just doesn’t do the job like it needs to.
You could, of course, make a test run both ways and see which turns out best, but who in their right mind wants to do that?! It seems to me that there are a couple of considerations. I’m not a pro, and I don’t guarantee that this is a foolproof method, but maybe it’ll help you reach a decision.
1) If there is already a dart at the bust, you shouldn’t have any problem with the pivot/slide method. Case in point? The Christmas Dress, it has a series of tucks and also darts, and took to the pivot/slide method beautifully.
2) If it is something you don’t want darts in, you could use the pivot/slide method, however, depending on how much you are adding, it might make more sense to just slash it and put in the dart, or try to “ease” the dart in (which I’ve never tried, but seen mentioned).
3) If your total added in the FBA is going to be greater than 1″/per side, it’s probably better to go with the slash/spread method. Nancy Zieman says that you can add extensions, if it’s going to be more than 1″ (and you’ll have to alter the sleeve as well), but for me it just seems like asking for trouble, not to mention even more tedious. I don’t know about you, but fiddling with the pattern isn’t nearly as much fun to me as actual sewing. 😉
4) If you don’t want/need the added length, the pivot/slide method (in my experience anyway) doesn’t significantly alter the length. This is especially important if you are attempting to match up tucks/seamlines.
Anyway, remember to take all of that with a grain of salt, since I’m not a professional or even anywhere close. But hopefully, it will give you some ideas on which method to use and when, because sometimes all these alterations can get confusing and overwhelming.