So, when I was messing with my own system before I came across Open Legend, I was mainly using d6 that exploded.
I actually did the mathematical permutation to figure out the average roll of a d6 (it was roughly 4.2, I had a more exact number, but I’d have to look it up now). This way I could figure out target numbers and the like.
You can actually figure out the mathematical average, b/c when it comes to averages, even though it can explode to infinity, based on the rarity, it stops affecting the total average after so many.