So, standard pitch rate is about 5-10M/ml for ales, based on strength, etc, right? I read Lars Garshol's book and
blog, as well as Milk the Funk, so they have suggestions, but just eyeballing it - my first Kveik was underpitched, about half to a quarter of the normal. About 1/2 a pouch of hothead for a 1.065 OG 5.5 gal brew, no starter, which is something like 3M/ml. Great esters, but took about a week to stop tasting green.
My latest was 1 sachet of Lalbrew Voss for a 1.056OG brew, 5.5 gallons, which is... Uh, I guess about 2.6M/ml, if my math isn't way off. I think It's tasting pretty mature already. No green flavors in the gravity sample. Not quite done yet.
When my wife and I
interviewed Gigantic for our blog last year, we tasted a Voss Kveik beer they made. I recall that they underpitched, but not the specific rates because I was super drunk at that point. She might have it in her notes. I feel like it was under 5M/ml, tho.
*Edit* - I also pitched at about 99f/37C with around 10ppm oxygen. Saw airlock activity within 120 minutes, did not rehydrate yeast.