Updating a numerical field by adding to it from another field

Hello

I have a numerical filed recording ‘total hours’ which I want calculated. I have another field in the same record recording ‘latest logged hours’. I wish to add the current value from ‘latest logged hours’ to the value in ‘total hours’. E.g. ‘total hours’ = ‘total hours’ + ‘latest logged hours’. Attempting to do this results in a circular reference error. Can anybody advise as to how I may achieve this sort of update?

Thanks.

Mike