Allocating Monthly Budget by %

Hello- I am working on automating a budget schedule for my construction projects. The budget allocation is large in some months, and 0 or low in others. I cannot use median to allocate a fixed amount across months as I would like that amount to be a % of the whole.

Is there a way to apply a ratio or percentage allocated over months? Also- the dates will change but spend should stay put (example: service fee will always be 100% on the 4th month of the project). Also- some budgets skip months. I have attached an example.

I appreciate all the help!!