To : P.G.Wilson
Generally , I don`t thing You will be able to find " narrow  operating
window " for batch type operation since the etch rate depends on combination
of lot of parameters :
a ) immersion operation : frequency and stroke of the agitation , extend of
aeration
b ) spray operation : type of nozzles , flowrates and pressures . 
>From chemical point of view : At low copper concentrations ( less than 2-3
grams/liter ) the etch rate is rather slow even at very high persulfate
concentrations . Above 10 grams/liter of copper the governing factor is
persulfate concentration . From my own experience , the process is not
highly sensitive to sulfuric acid concentration above 3 % v/v ( 6 % per
weight ) .
But if You really are looking after stable conditions , I thing You have to
establish steady state coninuous ( " feed and bleed " ) process .
I remember several years ago , in PC Fab.  papers on this subject .

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