So I bought Butterick 5726 fitting shell from Joann's for $2 to make a sloper, traced and cut the pieces, went through the FFRP fitting exercise, fixed the obvious problems, got lazy about the muslin, made one anyway, discovered different problems, lost patience, found it while taking a break, fixed some of those problems, refined the lines, and four weekends later - et voila - I have my sloper!
Here's the bodice:
I skipped the sleeves because I'm not planning to use them any time soon. Well, maybe some capsleeves... but, how hard can those be, right?
Hopefully all this exercise wasn't a waste of time.