Hi Phil,
I'm afraid varifocals are totally unsuitable for pistol shooting,my recommendation would be to buy a pair of pistol shooting frames from either Edinkillie or Intershoot,you can get a basic set for about £70,then book an appointment at your local opticians,take your pistol with you,and get them to measure the exact distance from your lens to the foresight,and adjust your prescription to suit,there really isn't a problem,I usually use Specsavers,and the opticians could not be more helpful,one commented that it was more interesting than the usual run of the mill stuff she did all day,and the lens only cost £15,make sure you get a glass lens though.
Neale
Steyr LG110 Hunter,AA410 in Gary Cane stock,HC, Steyr LP50,Morini 164ei,Morini CM84e,Anschutz 1417 thumbhole,Rimfire Magic 10/22,Anschutz 1913,Rieder and Lenz Z2,Keppeler 6mmbr