So the real arbiter is a multi meter. On my 85 which I am driving with regularity, the gauge always reads at half or below.
So does this mean that the alternator is not putting out what it should?
No.
On my car, after the timed lights go out and the car is revved to excite the alternator and I check the battery posts with the car idling I get 14.06v. This is with the gauge showing about 1/2 way or maybe a little below.
Turn on the lights and rev the car to 2500rpm, 14.2v, turn on the high beams and it drops to 14.06v again but the gauge throughout this is now at the 1/4 mark.
On my car the idle sometimes drops a bit more than it should so with the lights on and the idle low it will drop to below 13 with the gauge showing about 1/8.
So the moral of this is, use a multimeter to find out what is really going on and calibrate in your mind what the gauge is telling you, the absolute position of the needle relative to the gauge face is not indicative of the exact voltage that the car actually has available. If you really want to know I would suggest adding a digital gauge in place of the clock (or get Bob Brown to help you put one in the instrument panel) and run the wires to the actual battery where the load is being drawn from and replenished by the alternator.
The multimeter will tell you if you are running at a lower voltage than you should. If you are, I would measure back at the alternator to verify what is actually being output. If that is good I would measure back at the starter to verify a good connection between the alternator, the starter post and the main battery cable. Once you know those are good then the issue is back at your battery connections be it the positive or the negative. Taking each one off, cleaning it and applying dielectric grease and reassembly wouldn't be a bad idea.
Hope that helps at least a bit. The gauge lies, all gauges lie. They are just indications of what is really going on.