When we started our last Deposit Book, I sent a Finance message through MLS requesting guidance. They replied that my order was submitted based on my message. This was for the US so your situation may differ, but it is much more convenient for me to use the MLS message function rather than a telephone call because I am not usually in a position to call during their normal hours (even their extended hours are a problem for me).
My method is below in case it helps:
I keep the last set of everything in a separate location in our locked Filing Cabinet (Temple Recommend Books, Deposit Book, Deposit bags, Storehouse Order forms) that way, when I have to go get the last one, it is a signal that it's time to order more and I have time to do so without stress. We even keep a couple of bags of Sacrament cups in the Clerk's Office Cupboard just in case the last ones are used and they fail to notify me to order more (until Sunday morning when there are none!).