Low pads are indicated by...
Low pads are indicated by a gap between the string and the pad surface where the string leaves the scale.
Find at least two objects of widely differing weights, preferably with approximately the same footprint as a tire, say 3x10 inches. Place each on the scale in the location you will use for the tire, mark the scale pad and note both weights. (An interesting experiment here is to move the weights around on the scale pad and note any differences, if there are any. That will give you an idea of how important it will be to set the car in the same place every time.)
With the remaining scales, repeat the entire process from leveling to noting the shims required and the weight result. If all of the scales' readings match, you are ready to move on to the final setup. If not, try resetting the scales and repeat the process. Adjust and refine these readings until the readouts indicate a difference of half a percent or less. If you can't coax better than that, don't risk confusing yourself and undermining all the work you are going to do-have the scales repaired or replace them.
A Perfect Pad
Assuming you now have a calibrated set of scales, it is time to set up the scaling location. Pick an area that looks flat, smooth and is accessible to the car and a jack. Using the vehicle or a tape measure, locate where the scales must sit so the wheels will sit dead center on top of them. Roll the car back out and set the scales in position.
If this is the permanent, home-base scaling spot, or if you may be using it again, precisely mark the position of each scale to save the setup time the next time you scale the car here. You can scale your car elsewhere; you will just need to repeat the calibration and leveling process before you can attend to the car.
When the strings run straight...
When the strings run straight off the surface of all the pads without either gaps or bends, the scales are level and ready for the car.
Now it's time to level the pads. Remember that the pads should be leveled independently as well as in relation to the other scales. If one pad is higher on one side than the other, setting the car one inch one way or the other on the pad, it may make as much difference as a round or two on the jackscrew.
In lieu of a laser leveling system, tape a string to the center of the leading edge of the scale. Run it across the top of the pad to the center of the farthest edge on an adjacent scale or wheel platform. Pull snugly and tape the string to the edge. Now do the same thing from the side of that scaling platform to the side of the opposite. Repeat the process until you have a box of string connecting the four pads and forming an "X," dissecting the top of each pad from front to rear and side to side.
Looking carefully along the string, it should rest lightly across each pad without any space. If you find a pad or pads with a gap where the string leaves it, these are the ones that need to be raised. Look to the opposing scale where the string drops most sharply off the edge; that's probably the high one. Take your carpenter's level and level the high pad from side to side and front to rear using the shims. Again, be careful to support all four corners of the scale so it doesn't rock.
If the string still drops off the edge, it's time to move to the other three pads and level them. Once they are leveled side-to-side and front-to-rear, slide shims under the whole pad until the string lies evenly across the top of the platform at either end. That should bring the string even with the top of the high scale as well. It would be wise to note which shims went where so you can duplicate this setup quickly for all of your future weighing.