What percentage of memory should minecraft use?

I just bought and built my first ever desktop and I played minecraft on it for 2 weeks or so and I'm now getting a lot of lag and I have noticed that minecraft is using 78 and sometimes a lot more and a lot less like 35 or so memory and the memory allocated is at 100 or 40 or 35% and I found a website on how to allocate more RAM to minecraft and all I can allocate to minecraft is 1.5G and I know that I have more RAM that I can allocate to minecraft My PC has 8Gs I have no Idea what is making my minecraft game so slow can anyone help me. If this doesn't make and sense I'm sorry for making myself unclear I can't read and type what I'm typing to save my life. I would love the help thank.