Unless there's some special additional component in the Baqua peroxide bottle, you don't even need to mix something up. Just dump in 2.5 quart bottles of the 3% peroxide you can get at the store and you've put the same concentration of peroxide into the spa as you would with the Baqua bottle. When you're talking 400 gallons in the spa, having to add an extra quart of liquid makes no difference. Baqua would likely take a dim view of this and may have the standard warning about the use of other than their products violating whaterever warranty they provide, so that should factor into your decision. I've never used Baqua products and have no desire to do so, so I can't help on that front.
Starlight