"Solo" Sag Scales (single person required)- I have seen a few mechanical scales on the market that claim to allow the rider to set their sag with no assistance but to date, have not found one that is even close to accurate. In fact, the whole reason I designed Slacker was because I bought an ASV Solo Sag for $125.00 and by the time I got through the user guide it was obvious it could not possibly be accurate. The problem is they require you to set a slider, sit on the bike and then get off the bike to see where the slider is to get the measurement. This means you can not bounce the bike to settle it or the slider will measure to the farthest point the suspension was compressed to which will throw it off by up to 15mm! Likewise, if you have to sit on the bike gently to keep from having the slider get pushed too far, you have not settled the suspension and every time you measure you get a different reading since it depends on how gently you sit down. Worst of all they attach to the rear fender and that is only a good reference point for 2016 and newer KTM and Husqvarnas. The basic premise of these is flawed and you will notice these are slowly getting pulled off the market as the industry has become more aware of how important it is to get an accurate measurement that conforms to the bike manufacturers method. On any bike these scales are horribly inaccurate, not repeatable and very inconsistent. If you want to take measurements with no assistance, the Slacker digital sag scale is your only option. The other option is to get a buddy and a tape measure and measure according to the manufacturers recommendations which is a perfectly acceptable solution.
Folding Sag Scales (two people required)- These are an improvement over the solo sag scales but do require you to have someone to look at the scale to see the measurement. Basically, you place it on the bike and move the ruler on the top to it reads "0" at the point you chose to take the measurement and then when you sit on the bike, it will show you the measurement. The thing that makes these inaccurate is the fact that you cannot take the measurement using the correct reference points except on the 2016 and newer KTM and Husqvarnas. These scales are a good option for those two brands as long as they are 2016 and newer, otherwise you will not get a food measurement. The reason is, on any other bike you need to measure across the arc of the axle which means the correct reference point ends up being somewhere on the side plate and these are designed to measure vertically or some people will let them rest at the joint where the side plate and rear fender meet which is a convenient reference point but, not the correct reference point. Again, you may as well get a buddy with a tape measure and do it right since you already have to get someone else involved in the process.
For street bikes, neither of these are even and option as you are dealing with a molded body and they can't clip to the body and furthermore, there are no good places to measure to on the bodywork so these are useless for street as is obvious by the lack of any of them in street catalogs. Even a tape measure is not a good option for street bikes due to the molded body.
This leaves you two solid options- Get a buddy with a tape measure and make sure you follow the manufacturers recommendations to take the measurement or, get the Slacker digital sag scale. Either one will get you the right measurement, it is just a matter of the riders needs and preference.