Well, DarkZero, you've done great adapting your recipe, kudos on your flexibility, but math is still your Achilles slightly..lol I'm just buggin ya
Lets clarify tho..
30 US gallons is exactly 4 ft cubed
So 3.8cu ft is 107 Litres
Or 28.4 US gallons
And just 23.7 imperial gallons
To give you an idea of our style, which has been turning a couple heads for its health and easy add-water-to-the-end approach, even in one gallon smartpot 11 wk Sativas not to mention our 27 wk (16f) Sativa ..
With that said,
We add
one cup neem meal,
One cup alfalfa meal,
One cup kelp meal,
Thats..Per every 22 U.S. gallons or 84L as opposed to 28 or 30 gal batches
(Plus our other amendments and our rockdust recipe)
So you're mixing your batch up ~36% weaker based on my math. (18gal / 28.4gal = 63.37% or 36% less soil used for same amount of main amendment)
This might lower your yield a bit. I just did a side by side comparing batches, I wish I had went with Petros mix vs mine. Those amendments equalled fatter bottom nugs from the get go
We always here to help clarify anything if you decide to make a final last second addition of something..
Either way, best of luck man
-Tesla