No. You're not doing it right.
Yes. I have recommendations.
Take the total power consumption (Watts) of your system and multiply it times the number of hours you want to run it daily, Wh.
Divide Wh by 12. Multiply that number by 2. Multiply that number by the number of days you want to be able to run it without charging. That's how many Ah of AGM you need.
Use link #5 in my signature for your location, panel orientation and tilt. Pick the month with the lowest number of hours.
Divide Wh by the hours from link #5.
That's how much solar wattage you need.