9-iron tee height.

Flush. 9-irons play off the deck even on Par 3s.

The short answer

Flush with the mat. Do not tee a 9-iron up.

Why not

A 9-iron is a scoring club, and scoring clubs are about controlling distance and spin, not maximising carry. Both of those come from ball-first contact with a descending blow. Lifting the ball off the surface works directly against that: it adds dynamic loft, reduces the compression you get on the face, and takes spin off a shot whose whole job is to stop where you aimed it.

There is also a practice argument. You will almost never hit a 9-iron off a tee on the course. Practising the shot from a lie you do not play is rehearsal for a shot you do not need.

The one exception

A par 3 into a firm green with a tucked pin, where you want to take a little off it and flight it down, is a case where some players like the ball a fraction off the deck. If you are doing that deliberately, a quarter inch is the ceiling, and it should feel like almost nothing. Set the tee so the ball is barely clear of the surface.

What the mat is not telling you

Playing flush on a mat has one real risk, and it is worth understanding rather than teeing up to avoid. A mat will not let you take a divot. Strike behind the ball and the sole bounces off the rubber underlayer into the back of the ball, producing a shot that looks acceptable on the screen. With a 9-iron that error can cost you ten yards on the course and nothing at all indoors. If you want to know whether your short-iron contact is genuinely clean, watch where the sole marks the mat rather than trusting the number.

Distance control is the whole point

If you are building a yardage book indoors, the 9-iron through the wedges are the numbers you will lean on most under pressure. They are only worth writing down if the lie you hit them from matches the lie you will play from. Flush, every time.