Automated field update on saving a record cross table.

[GASkinner]

I have an app that tracks a running balance of camp sessions. A member buys say 10 camp sessions and then uses them over time by attending the camp thus reducing the available sessions.

I have a form that transacts the "use" of the session balance and can also "add" additional sessions when the balance is low or runs out. In some ways like an inventory of camp sessions.

I would like the form to update the "use" and/or "add" number when transacted. The form transaction is saved to a table of all camp session per member. Each member has a record in a separate table which includes the balance of sessions for the each member. 

I would like the form to update the camp balance for the member at the same time the add or use transaction is stored to the table. It seems the form is completed by the save button and I am having difficulty creating an automation to update the camp session balance in the member table.

Any help would be much appreciated.