InfoPath Dev

I'm trying to create a conditional default for a field and am having problems with the "else" clause.

 Formula:  concat(substring(outstanding_max, 1, not(not(Outstanding_Flag)) * string-length(outstanding_max)), substring(no_outstanding_max, 1, (not(Outstanding_Flag)) * string-length(no_outstanding_max)))

 Outstanding_flag is a checkbox defaulted to checked with a value of 1 for yes and 0 for clear.  Outstanding Max and no_outstanding_max are both textboxes that resolve to 30 and 20 respectively.  My problem is that I can't ever get the false condition (2nd substring) to fire.  Whether the checkbox is checked or not it always fires the first substring.

I would use conditions in a rule maybe place it on the Outstanding Flag check box and make one for true and false if necessary and pull the check out of the formula.

So create a rule with a condition OM=1 setfield with formula where you replace not(not(Outstanding_flag)) with 1

Hope that helps