|This tip is mostly for beginners are others who are new to fitting. IMO the easiest fitting book to start with is Nancy Zieman's Fitting Finesse.
Here is a tip for using the Fitting Finesse book:
Nancy Zieman recommends choosing your pattern size using your "front width" (armpit-to-armpit) measurement. I've found this is a difficult measurement to take accurately.
An easier way is to measure your high bust (all around the body, around the back, right under the armpits and over the upper chest area--not the full bust). Take it snugly and close to the skin. Use this number to choose your pattern size. If you are between sizes, use the smaller size. For example, my full bust is 38 but my high bust is 35 so I choose size 12.
And don't forget, if you make a full bust adjustment you probably also will need to increase the dart. Remember to check that part of the book when making bodice adjustments.
(P.S. the advice to use "high bust" measurement for choosing pattern size is borrowed from the book Fit for Real People by Palmer & Alto.)