Most technical book shops can sell you a Haines Manual, which is a generic version of the workshop manual. Some say they are crap, I have always found they are simple and logical!
As far as a fuel gauge go's, I have heard this question asked before and thus far havn't seen a simple and cheap solution. My best advice is to fill up your tank, ride for 100klms at the speed limit then refill and measure how much you used, then use the Haines manual to tell you how many litres your fuel tank has and then calculate it's range! Then repeat process on a key in brain out 100 klm stretch and measure the same. It will tell you max and min expected ranges. From memory, you will be looking for a fuel station around the 170klm mark!