I have an 88 242 with a 128 or 123 gallon Fuel tank if I read the plate on it correctl,y my eyes are not what they were when I was younger. The guage has Green and Red and it looks like Green is for Full down to 1/4 tank and then Red for 1/4 tank to empty.
Is the guage fairly accurate? When I get down to Red (idling) should I still have a quarter tank? I plan to figure this out by trial and error but the boat doesnt get used very much. Someone described it as a "big ass girl" which it sure feels like with a full tank of gas, 4 adults and a Rotti at 6,200' when trying to get on plane.

Make sure the outdrive(s) is trimmed down all the way to get on plane. :-)

Not sure about your particular guage, but I'm pretty sure the gas tanks are all about 100g on 242s. Mine definitely is. Here's a pic the previous owner took of it...

106 on the 242 if i remember correctly, run it to 1/2 then fill up, figure it out

ya 106-107 gal on the 242 unless you had twins