Costume designer here: Finished size is just that—what you get when the garment is done. Suggested bust size is actual body size the designer thinks it fits. That takes into account how much ease they intended to have. It does NOT REFECT the type of fit you prefer. Put briefly, body size + natural fabric stretch or compression (like during blocking or wearing) + design ease = final real dimension.
After making your swatch and doing the wash and block thing, you can tell how much the fabric changed or did not change in dimension. If the change is negligible, just measure an existing garment that hangs the way you like it arounf the body part in question (usually bust or hip). Compare to your own body, and that tells you the design ease YOU like. Add that to your body size and pick the FINISHED size closest to it.
If the knitted swatch stretched or shrank noticeably, then you need to add or subtract the change amount to your BODY size, THEN add the amount of design ease you found you liked.