We are installing a 21x41x52 oval - which I am sure I will no longer regret when I actually get the thing up

Anyway, we are at the leveling stage. We have removed the sod and are at "it looks pretty level to me", meaning everything is within 2 - 3 inches of being level. My question is how do we get to "NASA space floor level". Please be more specific than you think is required. I just can't seem to wrap my mind around we used a 2x4 and a shovel.

Thanks so much - I have already learned sooooo much from this site!