Determine Main Contact Number

[ViciousTruth]

Hello all,

Trying to figure out the calculation to do the following:

In my database, I have a Customer table and a ContactLineItems table. I use the ContactLineItems table to store phone number and type (Ie. Home, 555-123-0001). Additionally, I have a IsMain field that uses a checkbox: the user can mark a phone number as "Main".

What I am trying to do is write a calculation that prevents a user from marking more than one number as "Main"....

Any ideas?

[comment]

Instead of marking an item as "Main", store the item's ID in a field of the parent record.