A variable RVR is the high and low of the last 10 minutes. You don't care what happened ten minutes ago. All that matters is what the RVR is when you join the final segment of the approach.

For a CAT 1 approach, TDZ is controlling. CAT 2, TDZ is controlling, rollout is required. CAT 3 to 600 feet, TD and MID are controlling, RO is required. Below 600 feet all three are controlling