With filling the bath just deep enough so the keel is not on the bottom and both end encased I was finding it hard to guess if the deck was level Port to Starboard so I hunted through every nook & cranny for a little round level I know I have (and didn't find) but did turn up an 8mm dia x 35mm flat bulb and with a piece of blind slat scrap wood and a bit of epoxy came up with this mini level that will lay across the deck with pencil lines to help find the center line.
Ps: the lines will be redrawn as the bulb moved whilst the epoxy was drying.
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