Argh, where's mathsmummy? She's the expert! I'll answer as well as I can, but I may get it wrong.
In my experience, it moves around. It started off sort of quite far up & back & was quite a struggle to reach, as it was far away & also required a bent finger to prod at properly.
Now it's much further down, but it's also difficult to know exactly what's going on (for me, at least) because TMI I'm internally quite swollen, which is apparently due to increased blood-flow - the walls of my vagina are all puffy & it all feels different anyway & I'm not sure if that's the same for everyone.
But now, for me, it's not so far up at all & very easy to reach. It's still slightly backwards, but not really noticeably so. I've heard people say that it should feel like the inside of your cheek, but it's not quite like that. In terms of surface texture, it is quite a lot like that, but it's sort of more cushioned, IYSWIM.
If you wriggle your finger around a little bit, you can find the middle of the ring. I think I could probably get two fingertips into the middle of mine now, but that's a new development. & if they baby's engaged, as I say, you can prod the small gap in the middle & you'll find a solid, round thing - that's the baby's head