optimum stock based on supplier terms

hi all

trying to work out a way to calculate what optimum ordering of stock would be based on a overseas suppliers payment terms. I need to factor in that all shipments need their value turned over within 120 days of payments being made (trade finance)

for some suppliers it easy based on payment are due around time the stock arrives so are 120 days but I'm not sure how to factor in deposits and have a formula to return the optimum stock in days in column G.
so need to factor in Deposit paid, time from deposit to shipment, final payment due from shipment , time in transit before I receive stock.

I have manually added some of the basic results but not sure how to make a formula to reflect it and also one that takes into account the early deposits/payments.