How to decide how much to dilute extremely hot chillis I have come into the possession of a couple of naga varieties. We don't know which ones exactly, but google search seems to confirm that I do have some very hot varieties on my hands and they may average close to 1 million Scovilles.

I was thinking of making some hot sauce, how do I calculate the right ratio of chilli to filler so that I don't end up with something that will kill me?

As reference, the hottest chilli I regularly use are bird's eyes.

Update:
As a point of reference for others:

I started off with 4 of my nagas (and 1 tiny home grown halepeno) to 7 deseeded tomatoes. I just cut the top and bottom off, then run my knife around the seeds, taking out the 'cross-hairs' structure as well.

I had other ingredients, but tomatoes were the bulk of the sauce. This yielded about 700ml of sauce. At this dilution I could feel the heat, but it was quite mild.

I then cooked it down until most of the liquid was gone. A reduction of about 50% in volume. This left me with a moderate level chilli sauce. It's got heat but I can still taste everything.
Given that you don't know what you have, I'd favor making up "more of less 100% chili sauce" or as close to that as your methods come, taking care, and not tasting it.

Then take one tiny amount of that (a drop, a 1/4 teaspoon, a mL) and dilute it in whatever seems likely in your estimation (I suppose based on "possibly 1 million Scovilles" and some reference sauce you like) to be "more than plenty" of filler (1000X? 2000X?), mix throughly, and taste. If not hot enough, add more, mix again, taste. If still too hot, dilute more, taste. Keep careful track and you'll have the correct ratio for sauce you can stand.