Not sure I understand the concern?.. the 3cm is (accumulative) ease added to the pattern based on the measurements. For example… I’m plotting out vest patterns that I did in Seamly2D, where say it called for chestscale +1 (inches) on the back and similarly on the front. Chestscale being 1/2 of the chest measurement. So for someone that has a 38 chest, the final vest would sew up to 38in + 4in of ease. The same applied to the waist. In other words the ease is just ease - in this case 4 inches. It’s not meant to be proportional. The only reason I see to make the ease proportional might be if you were making pattern for dolls, where maybe you would want to make it to scale. If you want more or less ease, just change the value. Or better yet, you could use a custom variable - lets call it @Ease. Then instead of hardcoding the measurment_x + 3cm… use measurement_x + @Ease (probably divided by 2 or some number of pieces) . That way you can globally change the ease you want in a garment by just changing 1 variable. Want no ease… just set @Ease to zero.
As far as average bodies… you normally draft according to the body type, or grade rules if you will. Of course you also may have to take into account someone who varies from a given proportional draft, and know how to apply direct measures to it, but that part just comes with years of experience to know how to adjust a pattern. And then there’s the whole thing of adjusting for attitude, but that’s another whole topic. Again referring to the vest pattern I’m working with… I had to draft 2 versions, 1 for a regular type (the ideal men size being 5’-9"“, a size 38, and 180 lbs, and 1 for stout- where the the difference between the chest & waist is less than 4in. With that being said, there are avg regular bodies, avg stout bodies, avg corpulent bodes, avg boys, avg women, avg misses, avg petite… etc. Which in itself is all relative to the time period - people are simply bigger on “average” today than 50, 100 years ago. You draft for that body type, with what ever ease you want / need in the garment. And that, again can vary depending on the era of the pattern system. A more modern system is probably going to add more ease in the draft. In any case, the ease is generally a function of style, use, or a persons individual taste. For ex: You could take 2 people with the same exact measurements maybe one likes 2in of ease, while the other likes 4in. Maybe you’re building period costumes for Annie, and the style dictates a tighter fit than would modern suits , where the ease maybe 2” instead of 4-6". In other words the ease has nothing to do with average bodies.
BTW, I might add a bit of advice. Regardless of of how you draft (or drape) a garment - on paper or in whatever digital app, ease no ease, tight fitting, loose fitting, etc… proper and accurate measurements are crucial to a proper fitting garment! It’s an issue I’m dealing with right now… besides the fact it can be somewhat subjective, very few people know how to properly take measurements - even some in professional theatre. Never ask someone what their measurements are… most men don’t know (their wife, girlfriend or mom probably does though :)), and women lie - oh I’m a size 6… yeah, more like a 16. Also asking for a (suit) size maybe irrelevant. For ex. One of the actors I’m working on measures a 47" chest… his form says he wears a 44. Maybe in 2022 you do, but not in the 1940’s. Again it’s that “ease” thing. The worst we get is when the teacher lets the high school kids measure each other. 
And I don’t care to know what you in the clothes you’re currently wearing measure - I’m not making clothes for your (ill fitting?) clothes (with maybe the exception of an overcoat).
. I cherish her wisdom and every lesson I have with her, her craftmanship and keen eye is invaluable for learning how to optimise a fitting garment, so I definitely will continue to learn that and built the experience.