I chaining to get a perfect Attack and Speed IV's (at least) Absol. The problem is that I have no access to the IV's judging function yet and therefore I have no idea if I'm getting the perfect IVs I want.
Same occurs when breeding, as I cannot check the values yet.
Also... How much SOS do I have to chain to gain a chance for hidden ability?
You can easily use Serebii's IV calculator to figure this out. The IVs it generates may not be perfectly accurate, but it should be able to tell you for sure when an IV is 31. Also, in terms of the percentages you were wondering about, this is a very helpful guide that I used when starting out with the SOS method.
Hope this helped.