I like the simplicity of just treating double single like a normal triple, it didnt work that well but a worthwhile experiment (unless this isnt the first time you've tried this)
Often you get a lot of obsidian from chests so getting 20 is not too hard. He only had 1 chest on each side of the bastion though. You also want to get 20 so that you can measure eyes more accurately. If you measure from the spawn you're always far away from the stronghold since they generate some distance away from 0/0/0