Ideally, you are supposed to do the D-rest when you have reached 75% of FG. For example, if your OG is 1.05 and your expect FG is 1.01, then 1.05-1.01=1.04; 75% of 4 gravity points = 3; 1.05 - 1.03 = 1.02; So you would do the D-rest at 1.02. 48 hours at 70ish, IDEALLY ramping up from your fermentation temp to 70ish at about 5 degrees/hour, then following the D-Rest, ramping down to lagering temps at about 5 degrees/hour.
When I do lagers, I generally follow those rules, but I'm pretty lax about getting everything perfect, and my lagers turn out great, but ideally, that's what the procedure would be. As a benchmark, 75% FG is generally right about when you start to notice the fermentation slowing down as far as airlock activity.