I estimated the entries based on actually extracting all Manx IGI entries that claimed to be based on parish registers or filled the known holes left by the updating - in total I guess 250,000 entries - Brian already has 140k entries in his burial index (100k of which were before 1875); each of Brian's indices to decennial census contained about 50k entries ie the total entries in the PR's for Baptisms and Marriages is equivalent to 5 census indices
Not an impossible task