If you want to be accommodating to people using different units of measurement, I would consider adding kilograms rather than stones.
It's weird, but I always use metric for 'small' measurements (a number of metres, for example), but for distances on the road it'll be miles. And I've used miles per hour for so long that I just can't get my head around how fast something's moving if it's quoted in km/h. (Of course, the fact that road signs are miles and MPH over here kind of drums that in somewhat...)
Likewise, when I'm cooking I'll work in grams and litres, but for body weight it's always been stones.
(Was born in 1970, by the way.)
1. add exactly 16.8 to the value in your head
2. open the calc app (at which point you might as well put "84 kg in lbs" to google)
3. don't worry about the exact result, give a rough approximation: "it's about 180-something"
#3 will suffice, people are usually looking for a rough ballpark figure. If it was important then I'd obviously convert it carefully, but I'm talking about when I've done a conversion when talking with a US friend in the pub or something where I'd probably mess up whether you add a tenth of the original value or the doubled value anyway :)