Allow for adjustments to % allocations for assignees on an issue

Description - Right now if there are 2 assignees on an issue there's no way to edit allocations. The only option is 50/50, and that is not great for capacity planning in WLB. 

Why is this feature important to you - It would make the WLB a more accurate representation

How would you like the feature to work - 1) adjust allocations within WLB, or 2) adjust % within the assignment area of the issue when editing.