Gas gauge/mpg, first Prius

Hello everyone,
About 2 weeks ago I purchased my first Prius, it is a 2007 Touring with almost 109,000 miles (I paid $5,450 - it was a great deal and under market by 1,400). I have wanted a Prius for quite a while and am grateful to finally have one.
So far I have had no problems and recently got an oil change, I have also been steadily researching online for efficient driving techniques in order to preserve fuel and battery charge. I did test the 12v battery tonight and found that it was at 12.4 (I believe, I tend to forget things quickly. I’m certain it was above 12.3).
I am averaging about 45 miles per gallon and am continuing to grow in my comfort levels and skill driving the car, but I do imagine it will take time to maximize efficiency. I don’t completely understand all of the technology involved with a hybrid engine as it does get complicated, nor do I have really any car knowledge at all - so this is a day by day learning experience for me all around. So far I am at least happy that driving the Prius encourages thoughtfulness, awareness of driving, and my relation to energy consumption & surroundings.
I filled up last Monday on gas for the first time, and today, one week later I am down to 2 bars on my gas gauge..which doesn’t seem to make sense, considering I’ve driven a maximum of 300 miles since I’ve filled up and have decent MPG’s. Before I filled up for the first time, my gas gauge did go to one bar, but then somehow bounced back up to 3 bars after I had stopped and turned the car back on. Apparently it’s not uncommon for others to drive even 100miles before their pips drop at all, this is definitely not the case with me.
Could it be that I need to reset the gas gauge? I have found a forum on how to do this.
Is there anything that I should consider doing maintenance on or that I should know? Any reason why my gas gauge would be falling so quickly?