It seems logical you could adjust it at either empty or full, but I believe adjusting at full is better. There are 2 important variables. Let me try to explain and I'll keep this as much out of the weeds as possible. I have mentioned a few times before the permanent magnet (the hourglass shaped metal piece) in your gauge will gain magnetism by being exposed to 12v. According to a friend who has many years more electronic education than I, that little piece is also going to de-magnetize just sitting there for 60 years, so its condition probably will be different today. But according to him, this loss/gain will not ruin the gauge but will change how it registers and that fundamental change will require the proper voltage/amperage to make it register correctly today. Let me repeat: the gauge will work just fine at both empty and full....as long as it gets the proper amount of juice and the proper resistance to ground (through the fuel sending unit). More to follow. Unless you know the full history of your truck and equally importantly the fuel gauge in it, you don't know what has happened to it along the years. Now, the gauge doesn't care nearly as much about voltage as it does about amperage (even though they are inarguably linked), but higher voltage will increase the emf (electromagnetic field) and it will increase the magnetism of the thin metal hourglass shaped piece. When your tank is empty, your sending unit tells your fuel gauge there is zero resistance to ground. When your tank is full, your sending unit tells your fuel gauge there is 30 ohms resistance to ground. If all is well with your gauge (and by this I mean it is adjusted so that 30 ohms resistance gives you a nice full reading), then you will have a nice empty reading when the resistance is zero. That is the way that gauge is made. If you have an adjustable voltage regulator and a fuel gauge which has some unknown degree of magnetism, you can actually watch as you adjust the output higher from 6v and see just when you hit F.
Inside the fuel gauge you have two coils...and they're different (they're even different colors). See image below. You have a choke coil and you have an operating coil. The choke coil pulls the needle (which is attached to the hourglass shaped metal piece I've mentioned) toward the empty side. The other coil (the operating coil) pulls the needle toward the full side. Electricity enters the choke coil when the ignition switch is turned on. That coil is connected to the operating coil...and the tank sender which provides either ground or resistance to ground, depending on the level of fuel. The other end of the operating coil is connected to ground. Okay, now if you have an empty tank, the electricity ends up going to ground through the sending unit. With all the juice going to ground through the choke coil, that coil is turned magnetic and it pulls the needle to empty. If the tank is full and you have 30 ohms of resistance at the sending unit, then most of the juice runs through the choke coil, into the operating coil and then to ground. This pulls the needle to full. The operating coil has more and smaller gauge wire windings, so it takes less juice to generate enough magnetism to over-ride the choke coil and pull the needle toward full.
So, assuming a good fuel gauge is going to work correctly if it receives the right voltage (which is linked to current...sorry, but basically it is) then adjusting to full (which requires you adjust through 30 ohms or whatever your sending unit provides at full) will set the upper limit. Adjusting only to ground misses this important part. And that is why you need to start with full...you may think your sender is registering 30 ohms at full, but years ago I proved right here not all senders will do this. In fact, I only know of one being sold which will do so predictably. If your sender is giving you 22 ohms or 45 ohms at full and you've adjusted things so that F registers correctly at that resistance, the 0 ohm resistance to ground (Empty) should follow naturally.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
