How to constrain formulas to certain records?

I have a many to many table: Unit_Schedules. In that table I am referencing 'Unit_name' from one table: Units; and 'Daily_Run_Time', 'Absolute_Hours_per_Week' (Duration and Formula fields respectively) from another table: Schedules. I have two additional fields in the Unit_Schedules table, 'Speed' (Choice) and 'Calculated_Hours' (Formula).

So a unit can have many schedules and a schedule can be associated to many units.  

How can I do calculations (in the Calculated_Hours field) on the 'Absolute_Hours_per_Week' constraining it to the records with the same Unit and with regards to the Speed also? For example: minus the 'Absolute_Hours_per_Week' for the Choice: high_speed from the 'Absolute_Hours_per_Week' for the Choice: low_speed for a particular Unit.