Adrian
No problem, just look at your long vision prescription and add +0.5 to that so if its +1.25+ 0.5 = +1.75, if you are short sighted it may be a minus prescription i.e. -1.25 + 0.5 = -0.75.
Don't forget to include your astigmatism in it if you have one.
Stephen Hing at Shefford will grind them while you wait and fit to your shooting glasses, or I've also used the Specs Factory local to here for lens's, if you have one, you'll have to see their Qualified person and explain why you want it different to your prescription as the ordinary staff are only allowed to supply as your prescription.
Have fun, good shooting
Robin