The 50% rule is a guideline to use when choosing batteries for a specific application. Once you have the batteries, it doesn't make sense to get hung up on a particular number, especially since discharging to 50% is only a little worse than discharging to 40%.
If you look at a Depth of Discharge vs. Cycle Life graph like this
https://ussolarinstitute.com/glossary/battery-cycle-life/you can see that the graph is relatively flat down near 50%. The difference between 50% and 60% DOD is about 20% of battery cycle life.
Just SWAGing from the graph, it looks like that particular battery has a claimed cycle life of about 950 cycles at 70% average DOD, 1050 at 60%, 1200 at 50% and maybe 1600 at 40%.
I'd recommend you keep a casual eye on your DOD to use the next time you need to choose batteries, but not to get too hung up on it. You pay the piper somewhere, either with a high battery investment, doing without power or more frequent battery replacement. You can upsize your batteries so you stay at 40% DOD and they'll last longer, but you'll pay more upfront. Every application has a "sweet spot" where it's most economical.
