Setting sag properly

I don’t think it really matters, as long as you do it the same way every time. You will need to experiment and see what works best for you/bike as far as sag goes

We always set it while sitting in the normal position you would sit while riding. Don’t forget to check static sag also. Very important to verify correct spring rate. 

Standing centered on the motorcycle preferably with arms at your waist.
Of course, wearing full gear with boots and helmet to get an accurate weight.

even with a slacker tool it can be difficult to get accuracy by yourself, ideally a second person holding the bars is what I prefer.

as mentioned, doesn't matter as long as you are consistent. Also, its just a number. Sag is a starting point. Set it at say 105, sitting down in a neutral position. Then ride. Go in a half turn, then try a half turn out. You will find what works for you. You may find 107 is best, or 103. If you go straight by the book at 105 you could be missing out. Its not a perfect science, just a reference point. 

I always used to do it while sitting but then saw videos of the pros always standing while they check it so I've done it that way ever since 

I’ve always done it sitting.  Now, having a slacker tool, I can see that sitting even a half inch out of place will change the reading.  So I think I’m going to start doing it standing with hands at my waist.  At least that way your weight will always be centered on the pegs so it’s as consistent as possible. 

I've always done it standing then ride a little and adjust it a little from there 

I finally gave up on my wife and when Motool Slacker or whatever it’s called came up on Facebook for 99 bucks I jumped all over it.  Kris Keefer commented in one of his videos about his wife helping him.

slacker tool is not bad money.

Sag is a tunable number and once you find the number that is best for you, you'll want to be able to match it everytime.

Sitting on the seat it's next to impossible to always sit in the EXACT same spot in the EXACT same position, so it creates a variable.

Standing straight up without gripping with your knees or putting weight on your bars on flat ground with someone not adding weight holding you will create the most consistent number.  You can measure stiction and factor that in also by pushing down and measuring, pulling up and measuring, and averaging the two numbers.

Here is a step by step guide... 
https://racetech.com/page/title/Sag

Measure it standing up with hands reaching towards the bars, or something to hold on to but a little fwd.

You use static sag to check that bike does not change ones you find the right number. On these bikes, you need 38-40mm, and preload ~4-8mm.

With my Slacker (have had 2) I always get 2mm less (edited) sag at the track than I do on my concrete floor in the garage. That also needs to be considered, also hot/cold shock. It means when i set it to 42mm in garage, I have 40 at track. Never fails and I check static in garage after every time I have practiced and washed bike, and at the track.

Preferably on concrete, you get 42/105-106 in the garage without gear.

30mm is way to little. On 2019-2022 and with SKF low friction seals in upper linakge, I had binding issues on both my bikes. Switched to Prox old style bushing and seals and it was all good again. It affected sag a lot.

(Edited, meant less not more)

30mm is borderline soft on the spring. I always shot for 30-40mm static sag hoping it is closer to 40mm than 30mm.