One of the more frequent 'complaints' I get from my students is understanding how to divide the circumferential numbers into sensible proportions for the Front and Back blocks. Most books will tell you to divide these numbers equally. While this method might work in a general sense, it will still require A LOT of muslin testing for fit, and thereafter tweaking of the block pattern.
If you then measure the adjusted pattern at various horizontal points (bust, waist, etc) you will find that the Front is bigger than the Back. In this instance, bigger means the length of the curvature from a fixed point on the side body to a corresponding fixed point on the other side. This difference is less at certain planes (eg. at the underbustline) than at others (the bustline).
For a woman's block, halving the numbers just doesn't make sense around the Bust measurements. Even if you are a modest A-cupper. Our rib cage curves out towards the front, encompassing our heart and lungs protectively in its calcium embrace. Unless a body has severe distortions because of scoliosis, humps and injury, the Front is always bigger than the Back. I must give a shout out to Kathleen Fasanella (who is, IMO, the Supreme Leader of Pattern Makers) for bringing this physiology to my awareness back in the day...
Over the years of measuring a variety of body types, I've worked out an average in terms of how much more to assign to the Front when making the calculations for drafting a bodice block. I was able to do this because I was mainly making close-fitting corset patterns where ease was unnecessary.
Now a basic bodice block is also a close-fitted garment. In my case, I make blocks for repeat customers from which I then can straightaway create new patterns for a specific design (assuming of course that they remain constant in their measurements). I will add the minimum amount of ease to make it just about wearable with a zipper. Remember that this is only a fit garment, not an actual fashion draft. The fit garment will sit very close to the skin without uncomfortably squeezing the rib cage, so some ease is necessary. This is my preferred method because any additional style ease can be added during the pattern design process of the fashion garment. I do this for all the body types that pass under my measuring tape, before expanding the patterns stylistically.
This method is something I teach all my drafting students, regardless of their experience. All will understand the reasoning and some will understand the equations necessary to draft the Fronts and Backs. Admittedly, there's algebra involved (because it is an equation and must work with any measurement inputted). Which means I lose the attention of about 50% of my students, after hearing them cuss under their breath and look at me with disappointment at my betrayal. So to make their lives slightly easier, I designed a spreadsheet with the equations already factored in. They literally just have to key in the full circumferences and length measurements and the spreadsheet will calculate everything like magic.
I also include this spreadsheet (Measurement Worksheet) as a download in my online course on drafting a basic bodice block.
If one is mathematically inclined, the worksheet can easily be tweaked to include other fields and conditions that suit specific work.
For now, the worksheet for the online course only specifies measurements for the upper body. As I add more modules to the course, I will expand the worksheet to other areas as well; namely the lower body and arms.
If you think this will help you in your learning journey in drafting, do have a look at what else I offer in the online drafting course (links below).
You get a free Versatile Blouse Pattern from my collection when you sign up through either of the above links, from this website.
Meanwhile, happy sewing and stay sane.