The biggest enemy when adjusting the diopter (reticle focus) is staying "in the reticle" too long... If you're looking at the reticle for more than about 2 seconds when adjusting for focus, that's too long. Our eyes are incredible at forcing things come into focus, we want to avoid this here.
Get your rifle into a comfortable position with the scope pointed at a clear blue sky. You want nothing else in the sight picture other than blue sky and the reticle. You don't want it set up where you have hunch or get into an awkward position to get a sight picture. I normally set my reticle focus before the scope is mounted; I have a portable mounting rig that holds the scope for me and let's me stand up to set reticle focus.
On the fixed 6 SWFA, which has parallax adjustment, turn that to the infinity setting. On the 3-9, there is no parallax focus so skip this step... As an FYI when I last spoke to a SWFA rep they said the parallax is "set" to 100 yards on the 3-9 HD's. This won't affect setting the diopter focus.
Look at the blue sky with your naked eyes for about 5 seconds, immediately go into the scope and look at the reticle. It will likely not be "clear" to your eye. It will look "thin", "faint", or "out of focus". Loosen the tightening ring so the diopter can be turned freely. Make a 1/8th turn adjustment (45ish degrees) to the diopter in either the "+" or "-" direction. Repeat the process... What we are trying to do here is "get it close" and determine if we need to head towards the "+" adjustment or the "-"adjustment, before we make fine tuning adjustments.
If you notice little to no difference in the reticle clarity after the 1/8th turn, repeat the process and make another 1/8th turn adjustment in the same direction. You will find that with each 1/8th turn adjustment the reticle is either getting slightly clearer, or slightly less clear to your eye. You will then know that you are either heading in the proper direction, or if you need to be turning the diopter in the other direction.
Once you start to notice the reticle becoming more clear and bold with 1/8th turn adjustments, it's time to start fine tuning. During this entire process you need to be looking to the clear blue sky with your naked eyes very often before going into the scope. Remember to check very quickly for reticle focus and back out of the scope quickly.
Once you're happy with how the reticle is focusing, start focusing your naked eyes on objects at different distances. Look at something 10 yards away with just your eyes, look at something 100 yards away, and then go into the reticle with it still focused on the sky. See if the reticle is still in focus, make very fine adjustments to the diopter if needed. Next, look at the sky with your naked eyes again and go back into the reticle, it should be very crisp and take ZERO effort for you to see every line, bracket, diamond, etc. in the reticle. Switch back and forth between looking at 10 yards, 100 yards, and the sky and going into the reticle. Take your time and make very fine adjustments as needed.
I've done this process with two Fixed 6 scopes and two 3-9'snow. Locked them in, haven't touched them since. The reticles are very easy to see, especially when compared to how they came from SWFA. They were unusable in factory setting. They are now very crisp and look like a bold, black reticle; where as before they were thin, washed out, and almost "greyish" in color.
