anybody ever had to re-calibrate your fuel guage?

[BUXMAN66]

having my dash problems (guages) sorted out from the guy i got my tach dash from (a board member). and one of the problems i was having involved my fuel guage.took out my old dash set-up and the fuel guage was still showing the last reading of the fuel tank which was 3/8 of a tank.i have not added any fuel nor have i ran it out,so it still should be at 3/8. got my new dash and put it in and it was reading that i had a full tank. is there a specific way to calibrate your fuel guage? i was told that i would need to fill the tank up.....and then point the needle to full,...thus calibrating the guage. don't these guages have a tendency to"fall" by gravity? i don't believe i understand exactly,.... this theory of how to recalibrate the guage to match what you have in the tank.so if all of the components are working properly (sending unit, good un-broken wire from sending unit to fuse box, and fuel guage) shouldn't the guage show the amt. of fuel w/in the lines ("E"-"F")of the guage? do you need to give the guage the position to represent "FULL"? or should it do that itself? sorry the post is so long, just trying to fully convey what i'm wantin' to say! thanks for any and all replies.