Nested IF statements are often recommended to be replaced with a CASE statement
If(not IsEmpty(field A);"NA"; If(not IsEmpty(field C);"mmHg"; Self))
Is probably the nested If statement format (unsure)
What result should be entered if both Field A and Field C are not empty?
"What result should be entered if both Field A and Field C are not empty?"
It should be NA, because Field A is a "Not Applicable" checkbox that makes all the the fields below it marked as NA, so field A will have a check mark "1" at the same time that field C has "NA" in it.
field A notempty, field C notempty - means field B = NA
field A empty, field C notempty - means field B = mmHg
Case ( Not IsEmpty ( FieldA ) ; "NA" ;
Not IsEmpty ( FieldC ) ; "mmHg" )
That case worked perfectly, thank you SO MUCH.