For a popper I would agree with 1X or 2X so the tippet is stiff enough to prevent the last part of the cast from collapsing - it will be firm enough to turn over the popper. Myself, I wouldn't have a tippet heavier than 10# test because I want it to remain flexible enough that the popper (or bugger) has some wiggle.
Regarding breaking strength, it is sudden stress that breaks tippets, not a smooth direct pull. Last year at the fly fishing club we did a test on a leader with a 4X tippet. Bending the rod double (a less expensive rod), and using a fish weigh scale to measure pressure - we achieved a level of less than 2#. So you could tow a large fish or even weed/branch to yourself with 2# pressure, if there was no jerking on the line. With this in mind I usually use 3X (6# test) tippets for all bass flies exept the heavy poppers or Clousers/muddlers.
Cheers.