Im no 109 experten, but i have spent some time in them and have a soft spot for them.
How do you guys aim your tater?
I find that on average you need to lead almost twice as much as a 50cal armed aircraft. So if you're used of 50cals, then whatever amount of lead you think is enough, will only be half of enough for the tater. I would say you would lead about 70% more than what you would for 20mm hispanos too.
I have my conversion set at 200, for both tater and 13mm mgs.
Personally, cause i only use the 13mm to scare people into turning, i would set them out at 650 or so. Being nose-mounted, convergence isn't as much of an issue. I think i have my tatergun set at 280 - but again since its center-line mounted, convergence is not as bigger issue as wing-mounted guns. The only difference is the amount of 'loft' it dials in.
Do I need to have my piper through target?
That depends on the relative movement of the aircraft to your piper and your G loading at that point. If they dont appear to be moving relative to your piper and you're pulling 1G (eg not turning), then put the dot on em and fire. The more of either will necessitate a requisite amount of lead. But because of the relatively low muzzle-velocity of the tatergun you will need to learn to 'lob' your rounds at your opponent - think of it more as a motar than a lazer if you get my drift.
Is it better to shoot just the tater alone?
IMO, yes. I normally only ever shoot the tatergun. the 13mms are reguarly left untouched. I occasionally use 13mms to scare people a bit when i dont want to waste a tater on a low-probability shot. You only got 60 odd, so fire them one or two at a time - one or two audible "thuds" at a time. Your timing needs to be very good. Practice, practice, practice.
Like i said, im no 109 guru, there are plenty of others who frequent this board who may have differing opinions.