Next you need to measure the self-discharge rate of the battery and any "vampire" loads that can't be turned off, like a hardwired clock.
What I'm getting at here, is this is just math... if the battery needs a "top off" you should be able to measure how long it takes to drop to a certain voltage and then know how long it will take to top it off at 1.25A, by knowing what the discharge rate * time. Then it's just putting the same power back into the battery as came out... or kinda... see, lead-acids don't absorb power as readily when they're nearly topped-off, as they can when they're drained...
I suspect that Battery Tender is well above 12V by the way.
At least one battery charger company CLAIMS that because aircraft batteries are a different specific-gravity than car batteries, that you need their special airplane battery version of their automatic charger.
For the record, I've used the Battery Tender Plus for YEARS on non-aviation batteries, and it works well, and is on 24/7... but it has a multi-level charge curve and you need to know if what it does matches what your battery will do, since it's sensing voltage and current based on a regular lead-acid battery's characteristics.
Even so, those batteries do eventually die... it's not a de-sulfating charger, or very "fancy"... it just follows a set voltage curve expectation of typical lead-acid batteries.