Hours on Project per Employee / Month

I have time-data of employees per week. They are booking hours on Training, Projects, Meetings... so on. Now I need to create a meassure showing how many hours out of their total hours are spent on projects per employee. (Not all Employees are on projects) I want to select this from the Employee.Nr and view the effect over the past 2,5 months. Highlighting those emplyoees spending more than 75% of their past 2,5 months on projects compared to those spending less than 75% on projects - So far I couln't find a way to figure this out. 

The table looks somewhat like this:

As you can see both different weeks & same E.Nr occur multiple times within on month. Any suggestions how to tackle this one? Thanks in advance!