Yeah it's ~2.2 lbs to the kg, so when I have to convert I divide kg by two and then take off an amount that feels about right (or in the other direction, double it and then add a similarly gut-feel amount)
Yeah it's that one-tenth that I just sorta fudge, if you're converting 84kg to pounds then multiplying by 2 lands you with 168, at which point you can either:
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 :)