This post is 20mm setback and the posts I want to use only come in 25mm and Zero so I'm in a bit of a pickle here..

If thats the correct seat position and you plan on using the same seat zero setback should work.

Yep that's correct and same saddle. I just need some other eyeballs to make sure it wouldn't be too far forward on the saddle rails.

You can max out the rails, it's okay. The only thing you lose is the ability to adjust the saddle more in whatever maxed out direction. I suppose it'll be a bit weaker, maybe, but for literally decades I adjusted my saddle by sliding it forward all the way and seeing if it ended up in the right place.

i had a 20mm set back post and my seat was placed like that. i switched to a zero set back post and the saddle sits pretty much centre of the rails now.

It certainly looks​ like it would sit in the middle of the rails. I suppose I'll try it and see what happens.

If you can move the saddle 2cm back on the existing post, and the new post has a similar clamp mechanism, then you can achieve the same saddle position on the new post.

The clamp that you have in place right now looks to be on the narrower side of what's out there. Just be sure that the post you're looking at doesn't have anything too much wider. With those rails, though, I wouldn't worry about using all of the available portion of the rail that's straight.

Looks like either the 25 or zero would work barring some crazy peculiarity.

i am confused. aren't seat post clamps and saddle rails all standardised? if not buying a saddle would be a nightmare.

It should work. Easiest way to know though is measure. (assuming the clamp is the same length) If there's 5mm on the rails from the clamp to the bend backward, you can go with the 25mm setback. If there's 20 mmto the front before the bend you can go with the Zero.

All else equal, the setback is likely to be a slightly better ride.

They're not, particularly if you throw in carbon fiber rails. My Ritchey seatpost actually has several different clamps available for different seat rails.

Most all seat posts work with conventional metal rails, but the length of the clamping area, and therefore how much adjustment room you have on the rails does vary.

I'm glad you mentioned that because I'm likely switching to a carbon rail Romin and want to use a ritchey post. The problem is they come in either 25 or 0 for the carbon posts. I guess I could get an aluminum one in 20mm but that doesn't sound appealing. Then there is the issue of the interchangeable clamps. I can't seem to find any info on how this works or on which posts it supports.

The Ritchey 1 bolt posts have different clamps for different rails because they clamp from the side and carbon rails are often taller then they are wide.

The 2 bolt clamps that clamp from the top and bottom can accommodate most aluminum and carbon railed saddles, and should work fine with a carbon railed romin.

Ok got it.
I just saw the Flexlogic is 15mm setback which would be perfect I think.
https://ritcheylogic.com/road/seatpos...-seatpost.html