It can be frustrating... But being a bit methodical will go a long way to getting you there, riding and less foolin around.
Everyone's trying to get you close...
Do as Leisesturm (and many others) suggests, heel on pedal, adjust seat height to the point where you can pedal both front and backwards without rocking the hips at all - no higher.
And doing the cycling inseam measurement, per Iride01, is a good thing. Then using the .88 formula, you'll prolly find the heel on pedal method and formula will be very similar, maybe almost identical - just some confirmation. And it will also give everyone here some idea of you, relative to your height - always a good thing in discussions.
Seatpost slipping - Make sure the seatpost is CLEAN ! Many put way too much grease on the post - which makes overtightening happen...
Clean the seatpost completely, ALSO clean the Seat Tube throroughly !!! as far down as possible. If you don;t have Carbon paste, use silicon grease. A SUPER-light touch/coat on the post of grease is all you need... After I coat a post, I take a clean cloth and light pull it down the post to remove excess grease - then insert post.
Once you do the Heel on pedal method and find that height - I would wipe the post clean above the clamp, I small square of electrician's tape placed on the seatpost back just touching the collar - if the post slips at all, the tape will bunch/gather up and alert you to slipping... also if you decide to raise a few mm, it'll be an easy reference to where you were, originally.

The numbers on the back of the seatpost, just a relative reference - they don;t mean anything, just a reference to make it easier for slight adjustment. Every seatpost might be different, so no good to go from one to another, using it...

Leave the stem where it is, go ride for a few weeks, and then it might become a consideration.
The seat adjustment for a rider is the single most important adjustment made on a bike. Everything else as made relative to that.
Ride On
Yuri