quick reply...
avoid by 10nm if top upto 10 000'
avoid by 15nm if top upto 20 000'
avoid by 20nm if top upto 30 000' and above
give or take depending on the size, stage of build up and always TRY to fly on the upwind side if possible.
if you fly above it, make sure you have at least 2000-3000' clearence but its not advisable
hope that helps.