You should fill to the bottom of the slit caps when the battery is fully recharged. That leaves room for the electrolyte to expand up the cap slots when you next do a recharge. If you over-fill when cold, the fluid will spill out of the cap tops when you recharge at a higher sort of voltage.
Next is the higher sort of voltage. If you really crank up the voltage, to over 16.5 say, you will get spilling out the tops in a big way.
Reconditioning is a necessary thing at times, but easy does it.