Ride height?
 

So, in the process of going to 750/450 springs on bilstein hard s shocks on my v6 swapped nb. 1.25 front sway, stock nb rear. 205/50/15 rs-rr on 15x7 wheels. Suspension is otherwise stock. Car has been seam welded, frog arms, subframe connectors, butterfly brace, rollbar, etc.
Csp legal splitter and spoiler.

Primary useage autocross. But driven to and from local events up to 75 miles away.

What is the sweet spot for ride height? My guess is 12.5/13

Most around here use pinch weld height, you'll want probably 4.25"-4.5". I'd just follow the Supermiata race alignment.

Gotta measure and test. Supermiata found that the suspensions natural roll stiffness helps a lot at near stock ride heights.

I personally run 5/5.25" pinchweld on Xida Race with 700/400 and it is excelent. But my car is 98% a street car. (I would have gotten the GS, but they weren't out yet)

What ride height?

Ultimately, the lower your Miata is, the faster it will be in competition. The limiters are bump travel and ground clearance. If your suspension bottoms to frequently, bouncing over bumps instead of soaking them up smoothly, it's too low. Ground clearance for street driven cars is really up to you. Decide how much ground clearance you need to clear obstacles in your daily driving such as steep driveways, speedbumps, drainage dips, etc.

But really, read the whole page: https://supermiata.com/miata-race-alignment-info.aspx