Since Tom is a certified plant genius (seriously, I don't know if you've seen his stuff, but it's out on the internet on a number of different forums, and he uses plantbrain everywhere, it seems), I'm not going to ever think he's steering you wrong.
That said, why do you say 15ml, Tom? I use it as well, and the dosing instructions do say 1ml per 10 gallons of tank volume, so that would be in the neighborhood of 4 ml for the 38. I know you have excellent reasoning behind that recommendation.