Cumulative Staff Contribution Per Process Activity Cannot Exceed 100%

I have a spreadsheet whereby process activities can be attributed to the different staff members who complete the activity. Each activity is on a different row and up to three staff can be assigned to each activity, so for example if a level one employee sorts the mail on four out of five days per week I would attribute 80% contribution to the task to the level one employee. If a supervisor sorts the mail on a Friday I would allocated a 20% contribution to that staff group.

My question is how do I prevent someone filling in the spreadsheet from allocating a combined contribution above 100% to any task. This could be for example either putting a number in greater than 100% into one of the allocation columns or a total across up to three different columns above 100%.

The columns where I assign the percentage contribution are E,H and K. I would like to generate an error which says something like 'Cumulative staff contribution per process activity cannot exceed 100%.