I used 48x18 on (I think) 21mm tubs, this rolled out at 18ft 4 1/4".
That should have come to 18'6" by my calculation, you might have wobbled a bit when rolling it out, or the tubs might be a bit thinner than their stated size.
@Sam started
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I used 48x18 on (I think) 21mm tubs, this rolled out at 18ft 4 1/4".