I have seen a few rules of thumb, I haven't seen a simple rationale of how they are derived which makes them all a bit confusing. But all apply only for non sealed lead acid batteries.
For example this is the Windsun.com recommendation
"When using a small solar panel to keep a float (maintenance) charge on a battery (without using a charge controller), choose a panel that will give a maximum output of about 1/300th to 1/1000th of the amp-hour capacity. For a pair of golf cart batteries, that would be about a 1 to 5 watt panel - the smaller panel if you get 5 or more hours of sun per day, the larger one for those long cloudy winter days in the Northeast."
And here is West Marines
"As a general rule panels that produce less than 1.5% of a battery's rated capacity in amp hours don't require regulation. This means that a 1.5A panel is the largest you should use without a regulator on a 100-amp-hour battery. Regulators should generally be used any time you have two or more large panels connected to your batteries.
If you're concerned about damaging your new gel or AGM
batteries due to overcharging, you can add a small, inexpensive charge controller. These controllers, also called regulators, are rated by the maximum number of amps in your solar array, and we offer a 10A and a 25A version. Regulators are ideal if you can't be onboard to monitor
your electrical system
If you seek out self discharge rates you will find some values for a lead acid battery of between 4-6% per month up to 8-25% per month.
Another big difference.
What you want to avoid is overcharging a new 100% charged battery being hooked up on a sunny day yet and the same time prevent sulphation on a older battery by allowing it to discharge.
I cannot see a way to square the circle, I would recommend an inexpensive PWM controller with a solar trickle charger
as they are more efficient than an MPPT
in this application.