Accumulative Field Creation

Hello,  I am trying to build a field that's able to do an accumulative calculation.  We currently have an object called "Project" which are linked to another object called "Project Task" (Master -child relationship).  Within the "project task" object, we have a field called "Total spent (hours)".  In a project can have an infinitive number of tasks, what we would like to calculate on a TASK Level (field "Acum project completion percentage") is the percentage of project completion.  The calculation for the next example would be:  Project's Initial Budget (hours) = 30H Task 1 Total spent (hours) = 5H Task 2 Total spent (hours) = 10H Task 3 Total spent (hours) = 5H  Calculation: Task 1 Acum project completion percentage = (5H/30H)*100 = 16.6% Task 2 Acum project completion percentage = (5H + 10H/30H)*100 = 50% Task 3 Acum project completion percentage= (5H + 10H + 5H/30H)*100 = 66.6%

How could I build this logic?