Here is my gas guage    A dipstick! It is stored under the toolbag lid   on the forks and is marked off in one litre intervals calibrated for when the bike is on the side stand.
 Each litre gives the bike just over 20kms so no guessing when I need to fill up before a ride .
The lower mark is the reserve level 1.7 litres or about 35 km.
The top mark is 10.5 litres Full
 Simple and works well.

My "gauge" has proven almost as reliable as the sun.

When the number on top hits 30, I should have 3/4 or a tank.  When it hits 60, I should have a half tank.  At 90 I should have a 1/4 tank.  When I hit 100, I'm expecting to switch to reserve and I have about 1/2 gallon or ~ 25 miles before I'm walking.  

Where I ride with the stock s40 speedo when I hit reserve I could be more than reserve range from buying gas.
 I would be genuine old school then.

I hang a small cheap 3 digit combo lock from the bike. After every fill up I match the lock to the last three digits on my odometer. I can do around 100 miles before I need to hit reserve. Makes trips easy to guess at a glance.

I'd be setting the numbers 100 miles ahead of the last reading,  and fill up when they match.  90 miles ahead if your using the Raptor petcock , would restore your reserve to around 30 miles (stock) rather than the 20 the Raptor allows.

One guy on facebook tied a string to his gas cap.
tied a knot for the various levels.

Well I must say I've not read so much convoluted nonsense in a single post.

FFS get yourself a waterproof marker and write the last three milage numbers on the glass cleans off with a touch of petrol. I like to have an idea that I might be running out of petrol before it happens.



I store the pen  between the side cover and the tool box cover...

Hobby lobby has a white grease pencil. They call it a China marker.