The % setting display is only a voluntary code of practice that BACTA members are supposed to adhere to. Strangely, Quicksilver, despite being one of their biggest members, don't play along with this.
Again, the 70% minimum is purely voluntary on the part of the manufacturers. 70-80% will give you a bloody awful game and they're easy enough to spot and avoid.
Quicksilver low-techs (Electrocoins, Party Time etc) are usually on 92%, with the hi-techs on 84%. 10p play machines are around 78%. Used to be 88% for the hi-techs and 94% for low-techs, but they quietly lowered their percentages around 4 years ago, the same time that they stopped displaying percentages on the machines.
In the really good old days of tokens, they used to set their hi-techs on 92%. Ahhh... memories of Hyper Viper in Quicksilver Sheffield....
IMHO the Quicksilver chain has really gone to the dogs in the past five years - very little in the way of new games (apart from s.16 machines starting to pop up like the plague), cheapo decor. I remember when they were the best arcades in the country, but that was back in the token days. Nowadays chains like Shaw's offer a slightly better game. Nobles are not too bad, Shipleys are to be avoided at all costs for their dire percentages (e.g. 82% for a lo-tech Stars & Stripes in Solihull branch).