It is good to underestimate.
Your zoom is a push-pull correct?
I used to think my takumar bayonet was really blurry at 200mm, but then I realized that I was often actually shooting at 180 - 200, because when I was busy focusing, I would pull the zoom a tiny bit. Now I set SR to 150 or 180, and things are ok.
Basically I ask, what length am I trying to shoot? I set SR to be 20 ish mm below that. If I go below that value (say, try to shoot at 100), I just shoot without SR. It's really not that graceful but I'm sure the price was right
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