You should never use the electronic keystone adjustment on the projector. Judging from your photo, it appears that while your current alignment is fairly good you may need to rotate (yaw axis) the projector on the mount to moved the projected image just a very little to toward the right and then also change the pitch of the projector on the mount to raise the image just a little. Then after doing this use the projector's lens shift to center the image on the screen. If after doing this you find the top vs. bottom and also one side vs. other side of the image are tilted a little in the same direction (i.e., clockwise or counter-clockwise) then you can adjust the mount's roll axis to correct the image. Assuming the screen is a perfect rectangle, getting perfect (or near perfect) geometry of the projected image comes down to correctly setting the physical orientation of the projector (i.e., adjusting the pitch, roll and yaw using the projector's mount) in combination with the projector's zoom and lens shift adjustments.
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