Yes jim, iv never used a 15 foot box with 8 foot collar but iv used a 8 foot box with 15 foot collar, if I'm using the 8 foot box and 15 foot collar I normally just double what depth it tells me and its about right, you'll need to have a wee mess about with it and work it out so you know roughly how deep your going but you'll get used to it, they ain't spot on anyway.