A common mistake in measuring cycling inseam is to use too thick a book. I fear the o.p. is misinterpreting 'large' for 'thick'. Actually you want a fairly 'thin' book that can slide up past your *cough* junk, and nestle in the crotch. The real crotch. Tailors measure only to the bottom of where the boys hang, and the difference for someone gifted (Arnold Palmer enters the discussion) the difference could well be 2.5". I also believe you need to be barefoot when doing this?

By big I am thinking top to bottom. It should be around 1” thick.
I will stand either barefoot or in socks against a wall on a hard floor against a wall with the bottom of the book against the wall to keep it square and the spine against my crotch firmly.
I tried using a standard size hardcover book. Alone. It didn’t work.

The reasoning to use a book is that most book are 'rectangular' and assuming the wall is also perpendicular to a somewhat level floor, the length in the rider inseam will transfer to the wall without any angle introduced... so the same length, floor to book spine/mark as the rider has inseam.
using something like a dowel, which can easily be tilted, and may introduce a measurement error.
OP: which ever method you decide to use, they will all get you ballpark within 2-3 mm.
The key is to get that "ballpark" number (sorry, I just had to... LOL!)
It's quick, it's easy, don;t make it complicated...
The only issue sometimes is getting the mark at the top of the spine (which is hard up into your crotch).
Put the mark at the bottom of the book cover... then step away, line up the book cover on that mark, then make the proper mark on the wall at the spine (spine is what you push up into crotch), and measure.
The 'heel on pedal' method will get you to the same place...
Ride On
Yuri

I never got around to getting a good measurement of my inseam length.
Today was a very pleasant 70° so I went for a ride.
I noticed quickly that my knees were bent at the bottom of my stroke so I stopped and raised my seat. I rode a little further and they were still bent further than seemed right so I stopped and raised it a little more.
After that I coasted a little and with my heel on the pedal my leg was straight. I went about a mile and after a concrete joint I heard something and saw my knee was bent again. I stopped and checked my seat stem and it had slipped down. I put it back where it felt right and tightened the cam a little. It stayed after that.
The index on my stem was at 2 when I started. That is where it was in the pic where y’all said it looked too low. It ended up at 6 where it felt right.
I did 6 miles and felt great after instead of having leg fatigue. I think my seat has been dropping a little over time.
I still haven’t adjusted my stem height.

Back when you were posting in the other thread I did some Googling to try and find your bike. I never did. What year is it, do you know? Most of the Roubaix's I see online are newer and TBH yours looks way nicer than any of the ones that come up in images. If you bought it from a dealer they should be amenable to getting you properly fitted. Even if you bought it private sale, a Specialized dealer should continue to honor the implied warranty on the original sale, especially for a low cost operation like a sales fit. Quick release seatposts have a threaded adjustment on the other side of the cam lever and if yours is slipping you need to tighten that adjustment just a little so the cam lever holds better. In my city keeping the OEM q/r seatpost collar means the loss of a sometimes pricey saddle very quickly. You only do that once, especially when its a tandem and that seatpost and saddle were attached to a very spendy Controltech Stoker stem and Stoker bullhorns.

Pepperbelly, we're all goin back and forth between you're 2 posts, and things are overlapping and creating more confusion than necessary....
I would USE ONLY ONE THREAD for your further discussion of 'SETTING YOUR POSITION - as relates to saddle height, stem, handlebars - everything which relates to your position on your bike... without more confusion than necessary
just a thought...
Yuri

I called Specialized not long after I bought it used from a private party who did not know what year it was. When I gave them the serial number they told me it is a 2013 Roubaix Comp.
I also think it looks better than the current models but the new ones aren’t made for looks. They are pure performance. Mine is perfectly fine for me.

I will create a new thread rather than using these 2. They started as separate issues but they have become blended. It is getting confusing even to me.