How much gas in the tanks

[seance]

I'll be removing the oil injection on my 85 Ranger 370V with a Mercury 150, v6, ser # 6617258 as soon as I can figure out how much straight gas is in my tanks. I'll be using Quicksilver premium plus oil at the 50:1 ratio (3 oz oil to 1 gal gas). I tried to siphon all the gas I could out of the tanks by using the primer bulb, but I still have 2" of gas in 1 tank & 2.5" in the other. Both tanks are a 14 gal capacity each and are a translucent plastic type. So if you were to break it down, the tank is 10" tall and holds 14 gal, or at 5" should be 7 gal, or at 2.5" should be 3.5 gal. Might this be close enough and just add a little more oil & if so, how much more oil? Or should I try to siphon more or All the gas out. I might be able to take the fuel gauge sending units out to try and siphon some more.
I'll be putting on a fresh water sep/fuel filter, should I put 2 tbl spoons of oil in the filter before I spin the new one on? I'll be adding Mercury quickkleen to the gas also.

Thank you for your time.

[EuropeanAM]

The BEST (and perhaps SAFEST) method is to indeed remove the sending units, and siphon every last bit of fuel.

Remove water separating filter, replace or completely drain it.

Fill fuel tanks with 50:1 mix.

Disconnect fuel line at engine... place it in a glass jar. Pump primer bulb until a consistent blue color is noted in the fuel (indicating that you now have pre-mixed fuel available at line).

If the integrity of the oil injection system was in question- best to drain the VST (EFI models), or the Carbs, prior to reconnecting fuel line and priming with primer bulb.

[blackram]

I took the tank out of my Triton and completely emptied it. I then put in fresh gas and swished it around....then emptied it again. Replaced all fuel lines and rebuilt carbs and fuel pump. Also replaced fuel filter. I still have my oil tanks though, they work great. I did it because the gas had set too long and I believe caused a jet to clog