Sorry Mag7, I could have made that clearer, and KR kindly stepped in behind me and cleaned up my inarticulate mess.
Thank you KR.
As he said, you want enough oil so that the oil level in the Dutch oven is deep enough that the mash inside the pot is settled into it, making the heat transfer into the medicinal oil, but not so deep that the stock pot lifts off those risers.
The amount of oil you'll use will be determined by the amount of mash you have in the pot and the depth of your exterior pot.
When I blend my plant material with that small amount of olive oil I use I try not to add any more than the amount called for. It makes a very thick mash, and you never think it's enough oil, but it is. The cells break down and release the water and the trichomes release their oils and you end up with more than it appears you'd have when you started.
My first batch I added more to consol myself that there'd be enough. That was the last time I added extra. Since then I mash it up and go. Just like I'll be doing this morning.