Don't get hung up on having perfectly exact things. Remember the factory gauge cost maybe a buck or two to make and were never meant to be super accurate. Throw in the cost and loosness of the sender and things can vary a decent amout. This is why I suggest getting a decent meter to check things. If everything is cheap like the gauge,meter, high tolerance resistors, and so on, readings can vary greatly BUT the better the parts and test equipment, the better the results should be.That would be a good way to test a gauge still in the dash. I have a used one coming so I will rig up a test platform similar so that I can test it before putting the cluster back in the dash.
Just curious, how exact are the resistors? If you measure a 15.7 do you actually get 15.7?
I don't know how long ago it was but there was a member here on the site that was just fighting his fuel gauge system and I offered for him to come over to my house and I set up a table on the deck with a car battery, my meter, some test jumpers, some resistors, a known good fuel gauge, and I showed him test connections and we did this and that and he finally grasped how things worked. He said he was just not understanding how things worked but then once he saw the tests in action and with conversation of questions and answers being talked about he said that was the ticket to understanding things. It's sorta like learning to ride a bike.
On a 0-30 ohm fuel gauge like yours if I had an exact 30 ohm resistor, a gauge that was exactly calibrated to have it read exactly full with that exact 30 ohm resistance in place, then all would be good but then I'm unsure as to how it might read if let's say the voltage was low like at 10V and then what it might read if the voltage was at 15 volts. It might be close and that's going to be acceptable for a thing like a fuel gauge. I don;t think splitting hairs with exact stuff needs to be applied to this setup. If I do though do a test and things are really wrong, then there is an issue that would need to be corrected.
Just so you know, here is a shot of my meter measuring resistance through one test lead, through some test jumper wires, then back into the meter through the other test lead. You would think it would be 0 ohms but it was not. Wire and connections have inherent resistance.
0.2 ohms is not a lot but might be depending on how precise you want things.
Here is a setup on the deck when I was doing some fusible link testing. I had to add a battery charger due to shorting out a fusible link and this would draw down the reserve power left in the battery.
Just be careful that you don't let the smoke out of any wiring or parts due to wrong connections or short curcuits.