Friday Girl, our Alpha 35 FD, was charted as "Not to pass under Wroxham Bridge with less than 6ft 10ins on the height gauge".
Over the years I've gradually brought that down and accept 6ft 6ins every day. When we were once stuck the other side, after a night in Coltishall, I came back to find 6ft 3ins on the gauge at LW. I was about to tie up when another Alpha 35 charged through the bridge towards us. The pilot told me that he was only going to bring one more through but if I got it exactly right at high revs "she should go through"!
I shalln't be doing it again!
(Oh yes, as per Robin's method I'd measured our air height as exactly 2m)