How much sand should I add to my Hayward S166T Filter? It looks like it says 100 pounds but and added 50 and its half full and I'm afraid if I add the second 50 pounds it will be too full. Should I add the rest or leave it as it is

I added the 2nd back and it looks fine but now I have a new question, the guy at the pool place said I could use Vaseline for the o-rings but don't Vaseline dry and crack the o-rings?

No way any petrolium product will break down the rubber gasket!! This is the way some pool companies get you to come back over and over. If you want to lubricate the gaskets use a silicone lubricant only!!