How about relating ammo loads to the number of ammo bunkers that are up some what like they do to the fuel.  I would suggest adding ammo bunkers in this case.  Maybe  4 (small fields) 6 (medium fields) 8 (large fields).  Maybe the max reduction to ammo load could be 50%.  Also like having the ability to choose fuel and bomb loads you can choose ammo loads 25, 50, 75 or 100%.