The ratio of build/recovery weeks is something that you should experiment with. 3:1 seems to work well for the majority but 2:1 works great when the loads are high relative to what you’ve done before.
When I said to drop volume 50% that might mean dropping it 60% after a really heavy building period or maybe 40% when the load has been light. Volume is not really the best word to use here. Training Load which is a more subjective term (which is why I use volume).
The critical thing to keep in mind in a recovery week is RECOVERY. This week is not for building more fitness. It is for absorbing all the work you’ve been doing in the recent weeks. If you need 3 days off. if you need to cut training volume 70%, do it. Do any and everything you need to do so that you are ready to begin the next heavy building cycle. Don’t get stuck on some number like 50%.