Loren,
I have the same Northern Lights 6kw on my 420 and I have been guestimating .4-.5 gph..... but I haven't been loading it down completelyBTW... slightly off the subject... I put an 18" 'Max-Prop' on in Oct. and it seems to have increased engine fuel consumption by about .2-.3 gph at 25-27k rpm. over what the (as delivered) fixed 3 bladed prop did