Your legs should have a slight bend to them when the crankarms are vertical. The old adage of checking by placing the heel of your foot on the pedal- and setting the seat height so that you leg is straight with no rocking of the hips with the foot horizontal, will get you in the ballpark. It may not take into account if you have a thicker sole like an MTB shoe vs a road shoe, but it will be close and your legs will not be overextended when the balls of your feet are over the spindle. You should not have to rock your hips or strain.
I would set it back to where bike shop had it and ride it for a bit. If you rode with the saddle too high and are used to it, setting it correctly will feel odd at first.
Tzvia- rollin' slow...
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