I'm sure you've got enough experience behind you to get the sense for what your plant can suffer and still survive- certainly we've seen plants come back AFTER HARVEST even from just a couple new growth tips so indeed you can cut yoru plants back much more than 1/3, but I think that for a newer grower, leaving behind more plant material is a good insurance against losing the plant entirely because of over-zealous hacking up. I'm not saying you're worng by any means, only that newcomers to the hobby should be conservative.