It is based on what I see at a lot of the commercial nurseries in SoCal. I've used it for 20 years in an earthquake zone when we've had rollers that have knocked 12" of water out of my pool... and my shelves never budged
And you are talking about footings and mortar? Don't get me wrong but I think you are way over-engineering a few garden shelves
You are going to end up with 1500 lbs of shelving material to hold 300 lbs of plants
Just make sure your bottom course is level and in contact with a solid surface (I make sure mine are on hard dirt after I scrape away the top level of topsoil) and stagger your joints. I know you have already spread gravel but it is best if you build your shelves first and spread your ground material around the blocks, instead of placing your blocks on loose gravel. You maximize compression strength if all the block openings are aligned up and down... but again I'm not sure it matters for garden shelves; it's not like we are planning to rest a house on them
One thing that will help - make sure you stay low and deep, and no wider than 4' per support. Don't go more than 5 courses vertical - this will put your top shelf at 40" (plus board thickness). Any higher and it is difficult to lift larger plants on and off - plus your plants will be too high to view properly. Also make sure your shelves are at least 16" wide - which will allow you to place flat growing containers (like Anderson flats) and wide and shallow bonsai pots. It is easier to stagger the shelves by going one course shorter for one shelf, and then back up for the next shelf. This simplifies using your shelves in areas with shallow curves.
Buy some half blocks... I think they cost me $1.50 at the supply store