This is for a 2005 Sportster Custom, but may be the same for others.

The drive belt on my bike was so tight you could pluck it like a guitar string and play a tune, but the original owner said it was ok. Me being me thought i would have to check.
Having read what others were using to check alignment/tension i made up a metal rod with an old style solderless nipple that slides on the rod and you can easily tighten in place.
Checked the bike and it was way out of alignment.
I slackened off the axle nut and noticed that the nuts and washers on each side were different sizes so could not use these as a measure point from the end of the swing arm (i think 1 washer is 35mm and the other 40mm, and the nuts are also different sizes). Jacked the rear wheel clear of the ground and found the belts tight spot (yes, there is one), marked this with chalk and left this spot in line with the window on the belt guard, backed off the adjusters by 5 flats each side and tapped the wheel as far forward as possible with a drift on the axle washers/nuts.
So, with a vernier gauge i used a measurement from the end of the adjuster threads to the lock nut and this gave the same "out" reading as the rod did.
Now, the handbook says to measure the tension with someone sat on the bike. My wife could not hold the bike up safely and my English mates 5 miles away and my French neighbors about 7 stones wet through (7 less than me) so i sat backwards on the bike kissing the pillion seat and got my wife to read out which marker the belt now came nearest to adjustment window. This gives a starting point before using tension tool, you can also feel the belt in this position so giving a rough idea on tension. Why Harley cannot give a guide setting when bike is on it's side stand is beyond me. I adjusted the lock nuts until both gave the same reading and then checking again with the rod. This gives 4 points to check to and after a couple of small (1 flat ) adjustments i got it to read perfectly on the tension spring tool. My belt must have been way too tight so hope no damage done the bearings/belt. Double check tension after tightening axle up.
The belt does feel quite slack when the bike is on the side stand ( maybe 1/2 free play) but just me sitting on it tightens it up a lot.
The rods easy to make but tricky to use as you have to go around pillion brackets / brake fluid res' and would benefit from being braced on the corners to take out the small amount of wag.
Round off the small bent end to fit snugly into the dimples in the swing arm shaft, and mod the solderless nipple so that it sits in the axle dimples.

As i say, hopefully this is useful and my belts tension'ed correctly!