Maybe I was just lucky with my lbs. I bought another one online which was slightly too wide, so it's probably best to measure it beforehand in case another shops offers you a generic one. Not sure how to measure it in practice but you would have to measure the distance between the taper and the chainstay on an 'imaginery' 42mm chainline. If it's longer than the diameter of your chainring you'll be fine.
Maybe I was just lucky with my lbs. I bought another one online which was slightly too wide, so it's probably best to measure it beforehand in case another shops offers you a generic one. Not sure how to measure it in practice but you would have to measure the distance between the taper and the chainstay on an 'imaginery' 42mm chainline. If it's longer than the diameter of your chainring you'll be fine.