You should not be riding a size 51 bike with a 13cm stem. So factor in putting a more appropriate stem length on your bike when doing the conversion.
Good for you. I’m happy to see people doing unconventional stuff to try and get more aero. I do wonder, though, why you would swap out your P3C. What are you really going to gain going to the Look other than a more unique look (sorry for the pun)?
Chad
Sounds like you’ve worked through the math on it, so that’s fine. I suppose a 13cm stem is better than a pair of nut-cups (just kidding TJ). In any case, assuming your math is correct (and that my math on the S&R is correct), then the S in the low position should be a good match for you. There are a ton of stems available for the bike. There’s a Venn-style diagram of all the stem lengths you can achieve by rotating, using the different holes, etc. It’s pretty much impossible to not find something that will work, whether you want very short or very long.