OK, this will be no problem. The key field of your table is the email address and that must remain as a data entry field. Key fields must be data entry field and not formula field.
So you're on the right track, make a formula field as you suggest, and then use a Form Rule that will say
When the email address is blank and that boxes checked than the action will be to copy the value of the formula field into your email address field.
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Mark Shnier (Your Quickbase Coach)
mark.shnier@gmail.com
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