It's probably 253/1024. That's 15% at 153/1024 + 20/1024 per merit level.
I suppose as long as there's precedent for haste values greater than 5% getting an extra 1/1024, I'll buy 253/1024. Worst case scenario, that's within 3/1024 of both 250 (the minimum interpretation of 25%), and 256 (the cap). Spilled milk.
Way back in the day SE stuck to X*10/1024 as their haste value, but since CoP they've been using the closer % approximation much more consistently.
Also, I did some testing a while back that showed Haste Samba was likely 101/1024 Haste using Hpmedes in sea. (51/1024 + 10/1024*5)