spot mid rates

There is no rules that says that we should!! I can only assume that you are referring to exam questions on multi-national netting in which case we have used mid-rates when the question has said that it is the group policy to use mid-rates. The treasury function can use any policy it wants to use, but the question would have to state the policy (and mid-rates would be the most likely because it would be the easiest in practice to apply).

ok sir thanks

You are welcome :-)