I am an all for reef fan, been using it almost from the beginning.
The only sure fire way to know how much you need to dose is testing. You've also got to know that dosing AFR means upticks in alk are slow, 24hrs or so later you'll see the change, so you need to be patient, and you should be testing at the same time of day.
I liked the idea of AFR because in my life and head, water changes are laborious chores that cause me to be unhappy. So I really used AFR as a means to not do water changes. I'm not saying you should do this, that's just what I did.
I still feel like if you're okay with water changes and you do them regularly, your bioload is low, and water changes will easily keep up with your needs.