Weight downward on a pole is not a problem. Figuring worst case of about 600 lb./sq.in (NTS-1991) for wood, bearing on 1000 lb./sq.ft. soil (UBC, IBC); weight, even with XF's, cap's, etc. is well supported. Resistance to overturning (lateral resistance) is a big deal. Wind load, seismic motion, and conductor tension are the factors, and must be calculated based on location (wind & seismic), soil type, and other factors that our engineers just snowed me with when I asked. POCO's have standards that are well overdesigned for their areas and equipment, and are the closest to "rules-of-thumb" around.
I just set a 12 foot pole in a resi yard. The electrical inspector told me that ...i believe building code (not for sure now) for every 4 ft of pole you go 1ft deep. So i went 3 ft. deep seemed pretty sturdy to me.
As Energy7 mentioned above, There are a few variables involved in the seating of a pole. There is no way that there can be a formulaic way of calculating the depth for all situations. Depending upon where you are in the world and the soil type and so forth, It's a mine-field, it also depends upon what you are going to use the pole for. We have used the formula 10%+ 2ft, with standard poles and have had them fall over when the ground freezes and then thaws, causing frost-heave. My opinion would be, ask your local PoCo Lines Division. And don't, set the pole in concrete, you are just looking for disaster, if you get bad frost levels and the pole is under any sort of strain.