This will depend somewhat on the waters you expect to anchor in. Let's say never more than 50', for an example. You want to anchor using the standard 7:1 scope. That means you pick your spot and pay the anchor line out until you reach the bottom. You look at the marks in your line, every ten feet. The first ten feet or so is chain, easy enough to tell the depth there, call that color silver. 70' back is a whipping of silver thread to know where to tie off. If the 20' mark is a whipping of red thread, then at 140' you have a matching red mark in the line. You get the idea.
Now, let's say the bottom is at 36', right in between the blue 30' mark and the green 40' mark. You drop the anchor and there's an approximate 4' of rode in your hand. You can either just call it 40' (that's what I would do) or you could put a series of 10' black marks between the 2nd set of colored marks. You've measured 36' to the bottom, the 30' mark is blue. Pay out to the blue mark and continue counting black marks (6' x 7 = 42'). So, you count 4 more black marks plus one (just to be conservative).
-Will