FYI, retention here means retention when you review your cards. How much information you retain across all the cards will depend on a few other factors.
Your retention for mature cards can be lower than desired retention because of overdue cards. That’s one. But also because you do not rate your cards honestly or maybe because FSRS doesn’t have a lot of cards in high stability region.
Now, as for what desired retention you should have is hard to say until we get the simulator. But if you want lower workload, it might be better to simply use the desired retention value “Compute minimum reccomended retention” gives you. If CMRR gives you a desired retention that’s lower than 90 per cent, there will be a steady drop in “retention across all cards” (because cards are waiting for longer before coming up for review). But you might be able compensate for it by learning more new cards per day.