What length mast are you using downwinding,paddling

I've been told 75cm is the magic length..

yep +1 75cm

75 was the magic mark. Local chargers have been going slightly longer last year, depending on conditions and purpose (race or surf).

Use 80 for everything nice compromise

Some companies measure from board to the 'top' of the fuse (where the fuse meets the mast), while others (Code) measures to the bottom of the fuse. So a Code 75 would be closer to 70cms. Take that into consideration.