Hi Peeyush yes once you have written an if condition the lineitem cannot be used for input purpose,
Hence have one line item for input purpose, have one line item for the if condition.
Have final line item to specify if this should take input value or a calculated value.
For example:
1) 2% Rate, Principal is 2000, interest 40 would be calculated
2) We know that interest is 80, rate is 2% which are input now so the system should calculate principal 4000 and publish for you
Line items:
1) To be calculated Member: Values would be "Int, Rt, Pri" - These are list members used for calculation purpose and format is list
2) Rate Input
3) Rate Calculated: Here link the Int Input and Princiapl Input
4) Rate Final : If to be calculated member is "Rt" then Rate Calculated else Rate Input
Similarly have line items for Interest and Prinicipal too
Hope this should solve your issue
Cheers
Harish B K