While you're at it, you may want to get a couple different ones as well. Maybe a "terrestrial" formula or at least one that is very different than the first. Using two very different formulas side by side is another good way to hedge your bets. For example: P668 + banana and Western + coconut. Most tropical orchids come up on at least one of these, sometimes both.
Another variable you can explore is the time of seed exposure to the sterilant (chlorine, usually), and the concentration. I usually use a 10% (by volume) bleach water solution, but for some seed I cut it down to 5% or 6%.
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