Setting a proper rear sag is first step in proper suspension adjustment. If sag is not correct, bike won't handle as it is supposed to, more or less.
From 2008 to 2018, in KTM 690 Enduro R manuals there was a section on how to measure rear sag, with suggested sag values. This section is left out from 2019 690 Enduro R manual. And more annoying, suggested sag values are gone. There is general rule of thumb for dual sport and dirt bikes, that rider sag should be approx. 30% of total suspension travel. According to this rule, rider sag on 2019 690 Enduro R should be in 75 mm range (30% x 250 mm)
I will share few pictures from Owners Manual for 2014 KTM 690 Enduro R, explaining how to measure rear sag. It is important to pick two points, one on the rear end of the bike and other on the rear fork. Use same points for all three measurement.
First, put bike on the stand, lift it, so rear wheel is unloaded and lifted off the ground. Measure distance shown. This is unloaded sag.
Next, put the bike back on the ground and hold it upright. Measure distance between same two points (marked with B on the photo bellow). Than subtract B from A and you will get static sag. For previous model years, KTM suggested that this should be 18 mm. I just assumed that it is same for MY 2019.
And finally, sit on the bike, in riding position, and take same measurement again (C). Subtract C from A and you will get rider sag. For previous model years, KTM suggested that this should be in 70 - 80 mm range, which corresponds to aforementioned rule of thumb.
Sag is related to riders weight and spring rate. Adding preload to the spring can help to some extent, but if rider is too heavy, stronger spring is necessary. If rider is heavy, preload can be added to achieve proper rider sag, but static sag will be to small and bike won't behave well.
When I measured rear sag on my 2019 KTM 690 Enduro R, I found out that static sag was 36 mm and that rider sag was 98 mm. Dressed for a ride, I weight approx 100 kg, a bit heavier than imaginary KTM's average rider. This meant that I must add some preload to the rear shock.
Unfortunately, to (safely) adjust preload on these bikes, shock has to go out (unless you have aftermarket remote preload adjuster). This is a bit time consuming task, I had to remove side and rear fairings, air box, lift the tank and pull the shock out. Some shortcut may be possible, but I just made a coffee and removed everything. Here is how rear shock on my bike was set in the factory.
Note that my measurements bellow are not proper way to measure preload, I just recorder that to illustrate how much I moved the rear nut.
Adding preoad is easy, just loosen aluminum ring clamp holding shock nut and use tool from KTM tool box, one lying on the bench behind the vise, to tighten the nut, adding a preload to the spring.
This is how I set it, just pass the middle.
Luckily, my guesstimate was correct. Upon reassembly, my static sag was 13 mm, which is less than it should be, but, hopefully, tolerable. My rider sag was spot on - 75 mm. I will set compression and rebound on Normal and test the bike and I will continue with adjustments from there. Eventually, I may go for stronger spring, so I can achieve 18 mm static sag and rider sag in 70 - 80 mm range.
EDIT: November 16th 2019:
I tested a bike with added preload. It handles better and it is noticeable. When riding fast over rocky roads, rear end is more planted and it deflects less, leading to better traction. It is still not on the level of 1090 Adventure R, which is much more plush. Hopefully, I will make it better playing with compression and rebound settings. On the bright side, I am really happy with forks.