It depends, it's not that simple, which is why the '50% rule' emerged.
For instance, if you discharge to 75% and charge every day you may have, for instance, 350 cycles before your battery needs replacing, if you discharge to 50% then maybe 170 cycles. But need to charge every two days so the life is the same. By looking at the graph you may find discharge to 30% gives only 100 cycles but you only charge every 3 days. Assuming your charging has no cost obviously charging every day from 75% is cheapest but really each charge has a cost and the added inconvenience may have a 'cost' too.
Personally I'm a 30% 3 day kind of person. As i can count on one hand the sets of batteries i've been through in 30 years it works for me.
An interesting fact is that the time spent charging in my examples will be roughly the same because charging from 75% the current drops much faster.