Here are your questions and we'll deal with them individually if that's OK?
"Placement of harness lines: Is there an easy way to determine where the lines should be for a certain size sail. Or do you need to make one run to determine the placement by the pressure on your hands. Should the ends be two hand widths apart or less than that. "
As you change sails, especially if the sails are from different lofts/sailmakers, you may need to move your harness lines a bit to get a perfect balance for each of your sails, but it normally does not change a whole lot.
Bigger sails usually require larger booms, so if you have a couple of booms (or more) and you normally rig each of your sails on it's own boom size, you should not be having to change the harness line position alot once you get them balanced.
If you only have one boom, you may have to do a bit more moving the lines to get a good balance on various size sails.
I like to adjust the harness lines so I can take both hands off the boom momentarily and have the rig just stay in place.... it does not fall forward or fall back, and it neither sheets in or sheets out. Then I know my lines are balanced.
The front attachment point controls whether the rig tends to fall forward or back and the rear attachment point controls whether the rig is neutral, sheets in or sheets out when you remove both hands from the boom.
2 hand widths apart is OK, I run mine a little closer at only one hand width apart.
The issue with getting the attachments too wide is that the rig has alot of angular pull
and this can lead to rig control problems (catapaults due to oversheeting...getting jerked off your feet.... things like this).
"How does the position of the mast track affect getting into the foot straps."
Since modern mast tracks are quite short, unless you are running the mast foot extremely far forward, the mast track position should have almost no effect on getting into your footstraps. If you don't have your harness lines set so your arms are at full extension when you are hooked in and in the footstraps, you may need to adjust your harness lines a bit. Boom height also comes into play here.
"Being powered up is the most important part of this equation."
I'm not sure how being fully powered up would affect getting into the footstraps....
beyond if you do not have enough power in your rig to get planing, on your shortboards,
you won't be able to even think about getting into the footstraps.
If you aren't really powered up, you need a larger rig or more wind.
If you are just trying to get into the foot straps what is the best spot for them and how much of your foot should be into the straps.
This depends alot on the board.... do you still have the Speed Slider.... how many liters are you sailing.
If you have enough wind and sail size to get planing, then the all the way back and outboard footstrap positions will normally get you the max. performance from your board and rig.
Hope this helps,