Is my gas gauge working correctly??

[hotwls13]

I think it’s working, but thought it would read higher. My new to me 70 gmc lwb 1/2t 350/350 had a half done blazer rear tank swap when I got it. I think I have everything hooked up right etc. when I first got the truck when started I “think” I remember the gauge going way to the right. Found neither sender wire (ground or the brown wire) were hooked up. I hooked them up but only had like an inch of gas. At that point the gauge read a bit below E. Today I finall got time to get 5 gallons in it. Now it reads about 1/8?? Thought it would read more like 1/4 since I “think” it’s a 21 gallon tank.

Thoughts?