Agree with Jake's rough estimate- To measure more precisely use a long tape rule (or long line you can measure later), put boat in water and hoist tape/line using main halyard and measure to water- Add ~3" for the masthead casting and thimble on the halyard.

Point is- You will need a "lot"- So mast rake won't work, loosening forestay could be "iffy" (How old is your 5.2?? If it does NOT have the "captive ball" loosening your forestay this much could be disastrous!). Only reasonable options I see are using your trapeze lines tied snugly at the front beam to limit side "sway", use boom (as a "gin pole") with halyard connected to rear of boom (or mainsail up- but probably not best idea due to possible wind issues during this process) and mainsheet (may have to take some "purchase" out of the system to get enough "travel") to lower mast forward (will have to take side shrouds loose- boom connected via halyard to top of mast and to rear beam via mainsheet will control forward "tilt" of mast- Once you figure the distance out you could mark your mainsheet. Once under bridge just sheet in, reconnect side stays, raise sails, untie trap lines and off you go! Reverse process to return-

Other viable option is just lay boat over on side and walk it beneath- Make sure rudders are up, tiller is secured (bunjied or stuck under hiking strap, boards (and everything else loose) are tied to boat or secured (or take them under separately) and have one person at tip of mast and other at bow/stern-



Kirt