It seems to be a very sad fact that these repop senders are woefully inaccurate. I don't have the money, time, ambition or facilities to do testing on these. I wish I lived in a bigger area, with more of you guys around, I'd be glad to help work these out
First thing to do is make sure the instrument regulator and gauges are actually accurate.
To do that, go to Radio shack, and buy a combination of resistors that will give you about 10, about 25, and about 73 ohms. Don't use 1/4 watt, you probably want in parallel to give you at least a watt. Example, the 23-25 ohm, you can buy a package of 4 x 100 ohm, 1/2 watt, and wire all 4 in parallel. This gives you a resistor of 25 ohms, 2 watts
Buy a bag of clip leads, and cut three in half, solder the pigtail clips to your three resistor combos.
Now you can check BOTH the temp and fuel gauges.
Low/ empty, 1/2, and full/ hot. Leave the key on, and resistor connected for about a minute to let the gauge stabilize
If this checks out, You need to establish "empty" in the tank. This of cours may involve carrying around a (safe) can of gas in the trunk
Run the car out of gas, then add what you want for "reserve", say, a gallon
Now without moving the car, on the level, get into the tail lamp left kick panel connector, and separate. Take your ohmeter and carefully measure the resistance of the fuel sender at that point, whatever it might be.
This measured resistance is going to show you "where" the sender is
Now remove the sender, when you can, and lay it out so that (vise, etc) you can measure the float position. Hook up your meter and move the float until you duplicate your earlier measurement. THIS IS where the float would be in the tank with your 1 gallon (whatever) reserve.
Now devise a way (wood blocks, whatever) to mark the float location.
NOW bend the float arm until with the float in the earlier physical position, you get 73--75 ohms reading instead of your earlier reading. This SHOULD now put the float in such a position that when you have your desired reserve, the gauge will read close to MT.
"Full" might be another matter!!!!