You should use close to 1/2 a gallon
My snapper push mower and two MTDs and my craftsman use about 1/2 gallon per hour
So that's a 6hp two 14.5hp and a 22hp all use about the same
If the Exmark Metro is anything like the G1336" WB I had and it had a 13hp Kawasaki I would guess for your 1/2 acre lawn you would use maybe a quart of gas. Oh and thanks for the Congrats lawn mower fanatic.