How to decide how much to dilute extremely hot chillis
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.