Usually when working with an existing topography that is conducive to building a pond, there will be a certain elevation that is the least amount of work to create a pond of X acres. I would determine that elevation first, to plan your "normal pool" water elevation level.
The acre-feet of water in the pond design must also be calibrated to the upstream watershed, rainfall amounts for your area, surface use of the land in the watershed, etc. Too little water coming into the pond is bad because it will be difficult to keep your pond a normal pool. However, it is also bad to have too much
water coming into the pond.
You might actually be better off with a LARGER pond in the latter situation.
Here is a link to the old USDA Pond Planning Guide:https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs144p2_030362.pdf
That is a very useful resource at your stage of the pond design.
It is awesome that you have good clay below the topsoil!
(Make sure you stock the topsoil so that it can be easily placed over your clay final grades that exist outside of the pond. For example, the disturbed shorelines above the water level and on the backside of the dam. It is much easier to re-establish groundcover vegetation on good topsoil compared to compacted clay.)
Good luck on your new project, and welcome to Pond Boss!