Calculating a unique field

[aalrr1]

I want to generate a unique login from a full name (first, middle and last initial). If the Login exists (or the middle initial is missing), the middle initial should be changed to X; if that exists, change to Y; and then Z.

I am able to assign a variable to the first concatenation and assign to the Login field; however, I don't know how to test it against the existing entries for the Login field. If I set the field to be unique, the validation is passive and doesn't allow a calculation to change the field.

How do I accomplish this? Thank you.

[comment]

You are describing a recursive process, so you need either a custom function (requires the Advanced version) or a script.