I think I've heard in the past that there is about 4000 PRH's but that is probably just an educated guess more than anything.
We received 5,200 for Arsenal away so you would assume that is about the number on 3+.
Leaving about 28,300 for those in the 2+ ballot. (33,999 - 4000 - 5200)
We gave Arsenal (and presumably also Leicester) 5672 for the home leg, bringing our home capacity down to 47,722.
28,300 out of 47,722 is nearly 60% so worst case scenario is odds of 1/1.66 I work it out as.
Of course, 47,722 didn't go to both Leicester and Arsenal since they both went down to all members. I'd say a conservative estimate would be about 40,000 went to both. Making the odds of being successful in the ballot more like 70% or 1/1.4
Anyone feel free to correct me on any of that because I'm sure I've missed or overlooked something!