You probably mean 320w (357 -10%) ?

No no, 357+10% is what I guess his sea level hour power would be. I saw his heart rate for the attempt was quite low for him (like +-155) so I asked and he said there was still quite a bit in the tank for a sea level attempt, so I asked if that meant that 357W is what he expects to do in Bolivia because he expected to loose around 10% there.

Hello Joeri

his comment on Insta was :

"Yesterday I utilised the gold standard method of calculating your track FTP: riding real hard for an hour"

So, my understanding is that it is his real FTP (of the day), or nearly.
Minus 10% of FTP is not "real hard", it is pretty much sweet spot.

For the real attempt :
altitude will lower air resistance and power, but giving him a net increase around 1km, 1,5 or more. Depending his own reaction to altitude.
Temperature (30°C instead of 21°C) will give him some more distance.
Tappering + "real attempt" motivation can give some more also.