Try using percentage values from each Data file in the PctOfTotal parameter of the ISArcExtract function.
Example of equal percentages for 4 files (25% progress for each file).
Code:
if not ISArcExtract (0, 0.25, ExpandConstant('{src}\Data1.bin'), ExpandConstant('{app}'), '', false, '', '', ExpandConstant('{app}'), notPCFonFLY {PCFonFLY}) then break;
if not ISArcExtract (0, 0.25, ExpandConstant('{src}\Data2.bin'), ExpandConstant('{app}'), '', false, '', '', ExpandConstant('{app}'), notPCFonFLY {PCFonFLY}) then break;
if not ISArcExtract (0, 0.25, ExpandConstant('{src}\Data3.bin'), ExpandConstant('{app}'), '', false, '', '', ExpandConstant('{app}'), notPCFonFLY {PCFonFLY}) then break;
if not ISArcExtract (0, 0.25, ExpandConstant('{src}\Data4.bin'), ExpandConstant('{app}'), '', false, '', '', ExpandConstant('{app}'), notPCFonFLY {PCFonFLY}) then break;