Adjusting TPS & changing spark plugs?

There are some references to the TPS being a possible culprit in erratic acceleration issues but does anyone have a recommendation as to how the TPS is meant to be set. The mounting on the 99 UK allows the unit to swing through about 10 degrees and someone did mention that 0.4V should correspond with tickover, but not how or where that is measured. Anyone have any advice on this please. Also, how the hell are you meant to change the spark plugs on a Forester, I can't even get my hands down the side of the cylinder heads. There is a write up somewhere on doing this with an N/A but that doesn't seem to have as much plumbing as the turbo so removing the airbox is unlikely to free up enough space. Any help would be appreciated.