@Sophia, I was thinking that this was an issue I had heard of before. Please take a look at this discussion because I believe you are seeing the same issue. Specifically
The issue is not specific to the length of curves. The issue is the way you are attempting to use increments. An increment is a global variable intended to be defined and assigned a value before the pattern is drafted. Perhaps you can use your formula (including the length of curves) inside the constructed pattern itself. (e.g. use a formula including the length of the arm hole opening to draw the sleeve cap)
@Grace and @slspencer may have additional insight