Thanks, the plan is to put a patio slab under the drain and then cement
the
drain on top of it. The skimmer will attach with a bulkhead opposite the
waterfall.

How much below the pond bottom should the drain opening sit? 3/4"?

The bottom of the pond should be graded to be smooth with the top of the
bottom drain with the clamping ring on. It does not need to go any deeper.
If it is set too low, there won't be room between the cover and liner for
water and debris. It is always good to bowl the bottom to make it easier
for the debris at the wall to find its way to the bottom drain easier, but
if it is bowled much, it makes it difficult to walk to the edge to get out
if you get in for any maintenance.

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RichToyBox
http://www.geocities.com/richtoybox/pondintro.html
Zone 7A/B Virginia