I hardly ever used the 8" coil. 5" for trashy areas, 10.5" for clean areas and area coverage.
Go slow, and really overlap your swings. Target separation is great. Oh, and get a spare middle- and lower-stem sections, and permanently mount the coil to those. This way, all you have to do to switch coils is remove the 8" plug from the control housing, unscrew the cam, press the button, and detach the middle section complete with lower section and coil. Attach the other stem sections with the 5", attach the plug, power on and GB (you'll have to change it slightly as the small coil sees less total mineralization than the 8" or 10.5". I can switch coils in a couple minutes this way, instead of 15 minutes unwinding the old coil, detaching it from the lower stem, and then attaching the new coil and winding the cable back up.
HH from Allen in MI