Pretty sure its 10-20% so it ends up avging like 90% of your sed over 6 hits.

It's 10-20 percent.
What actually happens, is that the game calculates 10% of your SED for a weak attack, and then multiplies it with a 1.0 upto 2.0 modifier, with only 1 decimal! So, you get:
10% SED time 1.0/1.1/1.2../2.0

It is generally assumed that each decimal has a similar chance (about 9%) of happening, so that the chance to get a 1.0 multiplier is as big as the chance to get a 1.9 multiplier. It's not something I'd guarantee though, since a minor variety would be very hard to spot without taking a few ten thousand samples.