The boot on deck has a small thin plate that is about 8-10 inches out from the the mast. If you caulk around that plate, even though it may not look like the culprit, it should stop you leak. At least it did on our 1987 38 IP #40.
I can not remember his name, but there is an Island Packet rental place in St Thomas, USVI and he gave me the advice. I just recauked after 5 years with no leaks.
Good Luck'
George
George & Pixie
Silver Sea
1987 38IP #40
Cruising the Caribbean