Oil Pan Question - how straight is enough?

[Praxiss454]

I'm in the middle of rebuilding the 230 in my 65 GMC. I just went through and tried to straighten out the bolt holes around the oil pan, and they individually look okay now -- more flat than before -- but when I set the pan on my flat workbench, it only makes contact at the 4 corners of the pan near the larger 4 bolt holes. I can see some light along the entire edge down to the other end.

So my questions are
1. How straight is straight enough?
2. If this is not straight enough, how do I straighten it further?