Adaptive forecasting based on variable project lengths

Hello! First post! ATTACHMENT BELOW!
I'm afraid I don't know how to describe this problem in one line - I think it's more complicated than I've made out in the title. I hope you can help!

Back story: I'm dealing with projects and basically I want to forecast (*by month*) how much people are going to cost. First of all, I list each person, how much they cost, how many days they are forecasted to work and then a total. Then I take a total of everybody, simple. At the top of the sheet I have a start and end date of the project - this is variable but will never be longer than a year.

So I've got a total cost figure and a start and end date. Separately, I have the average number of days that are worked per month (fixed) to help give me a more detailed forecast by month. For example, people work less at xmas and easter so those months will cost less. If every project I had was 12 months, I could just multiply the total cost by the percentage of average days worked per month - only I don't always have 12 month projects, and they don't always start in April (tax year start)

The problem: my issue is that I don't know how to make the forecast adapt to differing project lengths, and how to match up the correct months (and thus taking into account the corresponding average days worked per month). They can start anytime and last anywhere up to a year (first of April earliest start, end of March latest finish). I also don't know how to match the starting month to the correct one in the forecast.

I apologise for the awful description, I'm frustrated at myself for not being more articulate.Example1.JPG

Please feel free to ask questions.

Thank you in advance.