You need a Fluke Chugmeter.
I also have an optima which I charge every 12-24 months. I also have a great charger which handles that condition and goes into a 'repair' mode giving the battery periodic high current blasts to gradually bring it to 100%, mine starting from 4v up to the proper 6.4v.
$37 at Amazon "
GOOLOO 6V/12V Smart Battery Charger and Maintainer 6-Amp Full Automatic 6-Stages Trickle Charging"