Ahh right, Windows 8... I agree with you wholeheartedly... I won't touch Windows 8 either, and I'm sure my neighbors who I convinced to return their 'new' computer are happy they don't have to deal with it either.

due to my main 500 gig hd developing bad sectors ( 608 so far )
i decided just to go for it and pair the 40 gig ssd up with a 160 gig hd ( 74 gig for files downloads and the like and 74 files for aplications )

i installed windows 7
turned the hybernation file off ( 8 gig won there )
and shrunk the page file from 12 gig to 800 mb ( might as well keep that around just to be sure )

currently i have around 23 gig left on the main ssd
so we will see ho wit goes for oh say a month if i liek it and can cope i will keep it like this
if not i will upgrade the ssd

my ultrabook just HAPPENS to have a 256 gig ssd

I switched back to 7 (man, I really want to like 8, but its broken xD), and it seems to eat up more space (maybe because it has more updates?). IDK, but windows 7 + less applications and with no pagefile or hybernation at all (I moved the pagefile to another disk just in case it is needed), it eats up almost the 40 GB itself D: (windows folder 32.1 GB).
The speed is still amazing, but such big requirements can make a problem for your sdd size.
If your ultrabook has a hdd for data you could switch sdd's if you use a linux distro on it, but really, I am genuinely amazed about how much space 7 takes.
If you manage to keep about 10 GB free, I would say it's ok (25% seems more than enough, but from a pure "gut" perspective) and 25 GB free is a good start indeed (unless you need +10 GB for apps xD).

120 updates later and i have 17 gig free
im not too thrilled about having to juggle free space and the likes though
so i wil probably break out the torx 5 and a m2 backet adapter

i dont really use my ultrabook for much anyway
nothing i dont do with my ipad and the ipad is easier to move around