Thank you all for your generous and informative comments.
The videos that I have watched (more than once) are these Rio videos: Leader-to-tippet knots and tippet-to-fly knots. In both of these videos breaking strength is noted and each tier used lubricant on each knot before tightening. As noted, knot breaking strength varies significantly between knots. This has always puzzled me and Rio offered no explanation.
I just purchased a new fly line for one of my fly rods and will have to soon make a choice as to which knot to use between leader and tippet. I always use a needle knot (I believe that is the correct name) between fly line and leader. The new fly line may come with a loop and I may forgo the needle knot. Not sure about this at the moment.
My preferred tippet-to-fly knot has always been the turle knot. From the videos, I was happy to learn that this is a very strong knot. However, the hooks I am using have flat or horizontal eyes. In other words, the eye is not bent up or down. IMO, the horizontal eye is adding additional stress on the tippet material.
There must be some 'knot research' somewhere but darned if I can find it.
I think your comment makes perfect sense and could well be the reason some knots break before others do. Thanks for that bit of into.
Thanks for all of your great commentary. Knot to pull your chain (pun intended), but nearly 90% of the food the trout eat in the San Juan River are midges. I'm a wee bit your senior and if I could not tie these tiny flies I would have to purchase them and that would definitely make me very cranky. I've been tying since I was a teenager and have probably purchased less than a dozen flies in my lifetime. To be clear, I do have to use a magnifier when tying but I've accepted the fact that my eyes are not what they once used to be so this works well for me.
Thanks for the info on the Davy knot. I had never heard of this knot. I should get out more. Ha!
Thanks again to all for shedding some light on this.