The exact formula I need there is: (-2*(NUM_CHAN-1)) where NUM_CHAN is a variable set on initialization and never changed again.
In my test NUM_CHAN is 4, so the Hex would be 0xFFFA for the source's offset and using this value it works. But I've tried using a 16bit variable to store the result of (-2*(NUM_CHAN-1)) and then using it for setting the value of the offset and it doesn't work. The weird part is that it doesn't even start. I mean when I use this variable the program stalls and I don't even get to see if it works or not or even the stored value in the EDMA register.