Good on you for worrying about the problem. However, there really is no point trying to get the fuel on a lightie accurate to the last poofteenth of a kilogram.
Why ? Because you are starting with an empty weight and empty weight CG which is nothing like that sort of accuracy. If the aircraft has been weighed by someone who knows what he/she is doing, and has exercised considerable care, you might, with a goodly bit of luck, get the empty weight good to, say, 5-10 kg and the empty weight CG good to 5 mm. On the other hand, I've seen quite a few a lot worse than that ....
How long ago was it weighed and how much rubbish has accumulated in the bilges and elsewhere since ?
Then, when you start loading it, how accurately do you calculate all the stuff you put into the aircraft, including the occupants ?
Are you starting to see a picture, here ?
If you have reasonably large tanks, you can invest in an hydrometer for a few dollars to check the specific gravity. If you really want to play with it, you can check the fuel temperature and run the appropriate sums. When you are measuring the volume, how closely can you reproduce the level which related to the last fuel calibration the maintenance guys did ? And it just goes on and on.
So what do I do ? I'm an experienced pilot, experienced consulting engineer, and regarded as one of the leading weight and balance guys in Oz.
I figure the volume as accurately as I reasonably can, considering the earlier comments. Either I check the hydrometer reading (yes, I have one in the bag) or, more easily, check with the local fuel supplier what their figure is (that is relevant to what they pay so they keep a good eye on things). Your answer will have an error, certainly, but the error will be within the noise of the total weight value for the aircraft and it just isn't worth worrying yourself too much about the last gram.
I see that my colleague, earlier, has made similar observations. He, also, has an engineering background.