Need help automating a process

[Timeizmonies]

Okay so for work I have to make a prorated amount of screens a client can have in a year.

Here's an example of what I have to do:

120 unit property at 100% PPU. Contract term begins 12/13/14 and goes through 12/31/15.
120/12 = 10 per month.
10 / 31 (days in Dec) = 0.3225 screens per day.
0.3225 * 19 days used in Dec 2014 = 6.129 or rounded to 6.
Prorated allowed for the first contract term is 126.
Annual allowed is 120

So in the spreadsheet I'll have a column for units, and a column for the start date of the contract. If the contract doesn't start on the 1st of month or the last day of the month I have to prorate it like the example above. So since the contract starts on the 13th I have to multiply the screens allowed a day by 19 which is 31-12 because there are 12 days of month they have not used to screen. I have to do this for each client so if another client starts in 11/16, I have to do the same process but for 15 days of the moth. So some contracts start on different days I can't just use a simple formula for the column. I have a current product were I have to prorate hundreds of clients and to do each one manually is extremely time consuming and tedious. I think a macro would be the best bet but I don't know anything about them.

 

[martindwilson]

its easy enough to count used days in a month tho'
=eomonth($A1,0)-$A1+1

 

[Timeizmonies]

its easy enough to count used days in a month tho'
=eomonth($A1,0)-$A1+1

What is A1 in your formula?

 

[martindwilson]

a1=start date