Me thinks your over thinking it.      

 Around here guys are putting 4-5 inch tile every 50 feet into 6" mains (because they are there) and draining WELL over 15 acres.    The calculator fella's get it right, but the other guys who just put er in, realize the "sponge" effect.

 You can tile to super suck the 7 inch rainfall your going to get in the next 12 hrs on your "small" area, or you can put (for the $) way more smaller  tile in and it will work good the first year, real dang good the second year, and perfect the third year on forever.  

 Lower that water table!

 Git what Im sayin?