I am trying to forecast the performance of a sales performance over a 12 month period based on their past performance.

This calculation aims to calculate the expected number of appointments a sales person will book.

The calculation I would like to achieve is as follows:

Sum total of appointments already made in 2015 / the number of weeks that have already passed in 2015 * 52

This calculation would give the total number of appointments the sales person would expect to secure based on what they have already achieved in the year.

Please could someone advise how best to implement this formula. I have tried using a custom formula however it is not obvious how to structure this, particularly being able to determine the number of weeks at any given time.

Thanks in advance.

James