Have not got to this yet, but I will try.
First part of such a question be it to add a call or to remove a call will be saying if you should buy a payer or receiver.
Without knowing details, if you hold a calable bond, it would get called and you would be screwed, this will happen if rates go down, you would need something that makes you money when rates go down, that thing would be paying variable and getting fixed since you would pay low and get high, that would be accomplished by a receiver swaption.
use inverse logic for adding feature… so here you secured some points by simply knowing what to do
now as far as calculations i have not seen this, so this is just from my head
say it would become attractive for the issuer to call your bond if rates drop to 4%, say they dropped to 2% and the bond got called and you must invest at 2% now. if you hold a reciver swaption that allows you to pay 2% and get 4% , the outcome is kind of teh same for you, called or not…
i hope this helps