How to tie my anchor line to the right length?

[Duggywuggy]

I have a raft. On the front of it is an anchor ring. To that I have attached my 75' anchor line then to a 4 foot anchor chain then to my anchor. That setup is for deeper water. But what knot do I use to shorten the line for shallower water? I don't have a "cleat" on the boat, just an anchor ring. I would assume that entails tying a secondary knot at a shorter distance on the line to the ring on the raft.