I actually tried a similar method, but for an unknown reason, it didn't work.
I created a formula field in the OT table, concatenating date serial and employee id. The field isn't unique because it is possible for an employee to have more than one OT entry on a given day (there are reasons).
I created a similar field in the OT Projection table, also not unique for same reasons.
I then created a summary field in the OT table, which would attempt to total the duration of all OT Projection records in which the two formula fields matched.
It didn't work. I suppose because the only way I could use a summary field was to create a relationship between OT and Projections, and they aren't really related directly to each other.
