Effective seat tube angle measurement?

Is it possible to figure out my effective STA on my Madone. I know my saddle setback, but don’t know if that is even part of the equation.

What I’m looking to do is figure out if I need a M or L in a TTX/Equinox.

I ride a 58cm Madone SL.

Setback: 7.5cm
Saddle height: 813mm

Just wondering if there is some kind of equation I use to figure out where my STA falls. On the aero bars I woud like to probably have my saddle nose a full 4 cm further forward so I can actually get more of my sitz bones on the saddle…right now it’s taint city and I have to call no joy after maybe 20 minutes in the position…reset …go back.