parpar.js -n -O -d pow2 -r "'+red+'%" -s '+getPar2Blocksize(foldersize)+' -m '+str(ram)+'M -o "'+d+'" '+files+'

red is usually 5
the blocksize is size dependent for me
ram part can be removed
d is the par2 filename
files the list of files ...

def getPar2Blocksize(size):
if   size < 100  : pblock=str(  384000)
elif size < 400  : pblock=str(  768000)
elif size < 2000 : pblock=str( 1536000)
elif size < 4000 : pblock=str( 2304000)
elif size < 8000 : pblock=str( 3072000)
elif size < 10000: pblock=str( 3840000)
elif size < 12000: pblock=str( 4608000)
elif size < 14000: pblock=str( 5376000)
elif size < 18000: pblock=str( 6144000)
elif size < 20000: pblock=str( 6912000)
elif size < 40000: pblock=str( 7680000)
else:              pblock=str(15360000)
return pblock

sizes are up to you
input size is in MB