how high can you fill a corny keg?
 

I added too much water after the boil, fermented, and dumped it all in the keg. It's pretty near the to the top.

Is there any downside/danger to this, or can I put psi to 30 and not worry?

If your level so high it is covering the gas-in dip tube you can get beer in your gas lines.

If the pressure drops, yes, it will push beer back up the gas line. You should have a check valve on your gas line to prevent that though. It's not ideal, but the first pour will take care of it.

I fill to about a half inch below the gas dip tube.

Thanks. It is far enough from the gas dip tube I think it will be ok.

So this begs the question... Where is the 5 gallon mark? Right to the lid? The end of the dip tube? Lower?

At this point in time I guess it depends on how many dents your cornies have. Haha. I've always assumed the weld line at the top dome. Maybe I'll measure one next time I have one empty.