What you are adjusting is the preload or ride height, not the weight carrying capacity. That is determined by the choice of spring.
Measure the fully extended length of the shock, eye to eye, and record that number. Let's say you chose stock length shocks and they measure 13.0" fully extended. Now you need to know how much total travel the shocks provide. This can easily be determined if you have shocks without the springs installed, but rather than disassemble your shocks just call Progressive and ask them. For the purposes of this post we will assume the shocks have 3.0" of travel.
What you want to do with the preload adjustment is to have the shock "sag" from the fully extended position by a percentage of the total travel, with the normal load on the bike (rider, any gear normally in or on the bike). I like to use 35% as the sag value on a street bike, but you can fine tune that based on your own preferences. What this means is that 65% of the total travel will be available to absorb a bump, and 35% will be available when the road drops out from under the wheel. If we use my 35/65 values, then with the rider and normal gear on the bike the shock eye to eye length should now be 11.95".
3.0" travel X .35 = 1.05"
13.0" - 1.05" = 11.95"
Measure the eye to eye distance with the normal load on the bike and turn the preload adjusters equally until you achieve the 11.95" figure on both sides. Record the number of turns and/or mark the shocks so you can easily return to this setting in the future. Now, do the whole thing again with the heaviest load you plan on having (passenger, luggage). Turn the adjusters until you once again achieve the 11.95" figure, record that number of turns and/or mark the shocks, and now you have your two settings.
As you can see, this can get a little tricky if you don't have a helper to measure while you're sitting on the bike, and the measuring can be a pain regardless. So here's a little trick I picked up from someone else on this site (sorry but I forget who posted this great tip): Take some plastic pipe (don't steal the plumbing from the spare bathroom, get new stuff at Home Depot, Lowes, etc.) and attach 90° fittings such that the distance between the two fittings is equal to the distance you are shooting for, in the example above 11.95". While sitting on the bike, you can slip the lower fitting over the bottom shock bolt and then gauge how much to turn the adjusters until the top fitting will slide over the top shock bolt. Much easier than trying to one hand a tape measure.
Jerry