Switch to vinyl & the low ch warning will go away.
Fwiw - My ch is 50 & its fine.
For the cya as it pertains to a spa, the upper limit of 60 isn’t terrible & is still doable for a spa but anymore than that becomes a bit impractical to maintain.
Anywhere between 20-40 is fine so long as proper chlorination is maintained.
If you notice, there’s two sets of recommendations,
Recommend- which is the broadest
& ideal - which is more narrow.
As with all poolmath recommended ranges they shouldn’t be blindly followed or necessarily stringently adhered to.
They are there as guard rails.
Like with ph - there is no “perfect” ph only the one that works best for your situation & isn’t off the rails one way or the other.
Some people kinda freak out when they get a not recommended/not ideal warning. If you’re one of those people or they just bug you, you can turn them off in advanced settings at the bottom of the settings menu
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Poolmath is really designed for pools but hopefully we’ll get some spa features in the future! Trust me it’s been very requested
until then we have to do a few workarounds.