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

I didn’t raise my saddle from 2 to 6 in one jump. I did it incrementally with 2-3 stages.

There is a pic in this thread showing the stem height. It is as high as it can go. I am thinking about lowering it 2 spacers which will be near the middle. I think I am sitting too upright.
If my wife can get time and not be exhausted she has promised to take pics and video. She manages a hospitals ORs so I cut her a lot of slack.

This is a pic of the markings on my seat stem. I don’t know if they are simply index marks or if they are measurements. They seem to be a centimeter apart. It is hard to see it but the index mark line for 5 is even with the top of the seat tube. I don’t know if it slipped or if I just misread it. I may have only raised it from 2 to 5, not 6.

another vote on the tape idea

I’m guilty of using too much grease on the seatpost - and not-a-surprise the seatposts have slipped on my bikes

actually first discovered this when glancing at pics I posted on this forum - the seat height appeared to be low and then realized the post had slipped

initially I used a small piece of black tape to determine if the post slipped - but recently began to use 3M red reflective tape (pictured below)

I will be watching the index number each time I ride.
I will also be doing away with the cam quick release and use a nut and bolt. It should be more secure and like someone said it will make my saddle harder to steal.

I really like the LEFTY ! I've been jumping thru also sorts of suspension hoops for my gravel ride... 2 yrs ago, a riding bud go the gravel lefty... He's big, strong, power rider and often rides it on the weekly racer ride. But some how I've never plopped down the dough for one - dancing all around the idea, without full success...
Which model/version/year is this one ?
Ride on
Yuri

Your Roubaix is a carbon frame isn't it? Be careful what band you put on that seat tube. I'm actually surprised that it had a quick release. I would have expected a band similar to the one on my Tarmac, that is just essentially a bolt and nut, but also has torque specs printed on it. Maybe they didn't do that back in the day yours was made. Also, if it's CF, even with a aluminum seat post, I'd think carbon paste more proper than grease or anti-seize.

I don't know whether or not the concerns I just gave you are valid considerations when getting a new band for a frame with a CF seat tube. However it doesn't seem right that just any clamp band will do.

Perhaps some others that know will say if there are.

It is a cf frame and a cf seat stem.
I will look into the clamp.

Working off of a theory that when buying second hand all bets are off, I Googled up some images of 2013 Spesh Roubaix's. None have Q/R seatpost clamps OEM. "Danger, Will Robinson. Danger! Danger!". Or not. But slipping (or stolen) saddle/seatpost combos are no fun. Especially when they are carbon. Pro tip: Those Q/R seatpost collars are designed to curve nicely around the seattube when closed. BTW for anyone interested this site has a sweet deal running on the o.p. bike.

Use carbon paste on your steerer tube and seat post so there is no need to overtighten either. Most shops have one use condiment packs of the stuff.