There are no P-charts , weight and balance info or any other limitations printed in the Approved Fright Manual for flight (hopefully) with an ag' overload, it's always been up to the pilots experience , both in general and on the airframe type, to decide how much he/she will take under the prevailing conditions from the given strip.
I think we all start with a conservative load and work up to a safe limit and thats never clearer than when spreading solids, after a few seasons and scares most people become comfortable with judging a safe load to begin with.
I've always worked on having 3 extras up my sleeve during the take-off/ initial climb (!) phase; a little extra power, more usefull flap (ie not drag flap) and a few extra knots during initial climb with the dump lever as a last resort, if things aren't going to plan as the take-off progresses these can be utilised as required.
Only once so far have I had to dump liquid on takeoff , however the process of using up my backups one by one makes the use of the dump lever an easy progression .
Trouble is I'm still human and it's an inherantly hazardous game we play.