How can we schedule payment exactly

[ksuresh109]

We have a problem with supplier payment term.
supplier payment term is 20% payment in advance, 80% after the material dispatch.
when we are looking to schedule the payment accordingly to the Payment terms in PO.
but while creating payment term we can give 20% adv with zero days, but for 80% we can not give exact days or date as we dont know when the mat. will receive in to stores.how 80% of payment will schedule to pay.