Well, I am little confused by your needs. But, reference the attached spreadsheet. First off, I had to convert the OT Worked formula to convert to decimals. That helped the overall maths.
The Working Day 13 or 15 is just a simple IF formula: =IF(I9>13, 15, 13)
As for the alert system, there are several ways to do it. In the example I put in, the Working Day column header is fitted with a conditional formula that counts the number of 15's and changes the header color based upon the result (green, yellow, red).
Hope that helps.
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