Pro-rata monthly budget by week

[blackduck]

I need to pro-rata a monthly budget so that a greater proportion is allocated to beginning of month.
They want it to be:
working days 1-5 = 40% of total budget

6-10 = 30%
10-15 = 20%
15+ = 10%​

There is a table currently in the system called x_workdates:
Seqno Date Weekday Workday
2071 1/09/2016 5 1
2072 2/09/2016 6 1
2073 3/09/2016 7 0
2074 4/09/2016 1 0
2075 5/09/2016 2 1
Any ideas on the best way to go about this would be great.
Thanks