Just read your owners manual and don't go more turns than the reccomended settings. I also have a 13 heritage and mine was way too stiff when I bought it.
I adjusted mine down with no problem. I just put a piece of tape on the cylinders so I tell how many turns I made. Also, I did it with the bike on the side stand.
If that's the case I'd ask the dealer why HD sells this,
Shock Adjustment Spanner-94448-82B | Tools Gauges | Official Harley-Davidson Online Store
The shocks on the 13 model are the same as the part # on the 08 Heritage I used to have and yes, adjusting the shocks for differing weights is a good idea.
I have posted on here about this same thing earlier. I REALLY have difficulty (read impossible) to adjust the shocks on my 2001 Heritage. I wont rewrite all the other posts here but I will re-state -- the spanner wrench would NOT turn without hitting the frame. Plus the holes are at 0-90-180-270, so you can ONLY turn shock in quarter turn increments. HOW did you adjust your shocks.
Thanks so much,
Bill