You can get 500g of Citric Acid powder for under £3 (e.g. Amazon.co.uk) delivered. I’m curious as to how much Mercedes-Benz charge for theirs…
Some how I think you will drain more than 3 litres out of the system based on the fact a gallon is 4.5 litres.
As the system is 7.5 litres.
I would assume Mazda’s Technology department have calculated that the FL22 has a lifetime of 11 years hence the change at that period of time. 
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Why anyone would feel a need to " flush " a system during a normal coolant change is beyond me.
Unless a system has endured gasket failure or dilution with water and resulting rust the system will only contain fl22 with a depletion of inhibitors.
Drain and measure the volume of what comes out and top up slowly with the same volume of fresh fl22 at 50%. Saves hassle and achieves the same thing.
Ravenol HJC is available cheaper than FL22 at 100% concentration via their website.
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Sorry
My bad
I should have said i changed mine at 10yrs because the ph level was lower than 7 @ 10 yrs and 42k
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I believe the change to 5 year intervals after the initial 11 years (or 10 years in the USA!) takes account of the enfeebled old coolant left in the system.
I’ll get a specialist to do mine because I also want a new thermostat and it is a little hard to access (throttle body needs to come off).
I just let the dealer do it. From memory it was £125 all inclusive.
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