Every aquarium I set up (I only have 4 currently), one of the first things to do is to paint with marker a small black line on the left edge of the tank, mid-way between top and bottom of plastic frames. This is my ~50% line (although it actually is slightly more due to substratum, not filing to the very top, etc.) This is what I strive for. This works for every size and shape of tank, regardless of volume. During water changes, I aim at going below that line by as much as I can without stressing the fish, messing the landscaping. I do this every Sunday, regardless of parameter readings, which I confess I could do more often. I estimate I am changing 60-70% of water once per week. I wish I could do this twice a week, but it is tough for me. It seems to work well enough.