No matter what you decide to do, make sure the pole is sitting on a solid tamped bed of gravel and n ot stuck into solid concrete, stone, or clay. Part of the code for pouring construction pads or footers is having gravel underneath to provide water drainage or overtime it can erode the base under the poured concrete structure. When using sonar tubes you are suppose to put a 2" layer of gravel under it for that same reason.
Myself, I would follow the 10% plus 2 feet rule, be sure to be under the frost line, and set it on gravel. Concrete optional. I will tell you I buried my 8+ foot ,1/4 wall, 4 " pipe, about 32" deep for a gate, and when I got my B2/two axle trailer stuck, I ended up using it as an anchor point and started pulling it out of the ground. I moved it a good 2 inches sideways and compressed the soil, before I decided to use a 20" tree instead about 50 feet beyond it for a block anchor point.
When I take it out of the ground to be reset, it is going to be 36" deep (frost line is 32") and 6-7 bags (50#) of concrete in a 8x8 or 12x12 hole, with #3 or #4 rebar, so I can actually use it for pulling.