1.As high as posible.
2.Dot in centre of crosshair at your chosen zero range.
3.There are but I had mine custom made and then put it on top of an inverted scope mount. Very high very cumbersome.
The way I had it set up was the nearer you were the higher the dot would be above the crosshair the further away the lower it would be.
This was on a .22 I personally wouldn't bother with a .177 although if you are lamping it will confirm whether you are closer or further than your zero point. The dot also can flare of different backgrounds.
I shot an awful lot of rabbits with this setup but I personally can't recommed it for .177 at ranges out to 40yds which if your lamping I would suggest is along way.
Andy