I had this problem.
Two suggestions -
1 - hoist a piece of string up to top of mast with a halyard and stretch it down to deck height and measure it. Then inside cabin measure down to where sea level is and add the numbers. Alternately to be really accurate you can stretch the string from top of mast it to the surface of the water and use a free online trigonometry calculator to get the measurement. In that case the length of the piece of string is the "hypotenuse" of a right angled triangle. See here ....
www.csgnetwork.com/righttricalc.html 2 - There's free "ruler" software you can download. If you can get a good side on picture of the Hood from somewhere measure how many pixels from bow to stern (=23 feet) then use that to ratio metrically measure the mast height.