Simple Python script for reading from a file, changing something about it with a regular expression, and writing the result to a new file.

parse.py

import re

read_in = open("original.plist")
write_out = open("new.plist","w")
minus = 280
pat = re.compile('\t\t\t\t<real>([0-9]*)<')
for line in read_in:
  mo = pat.search(line)
	if mo:
		value = int(mo.group(1))
		writeme = "\t\t\t\t<real>" + str(value - minus) + "</real>\n"
		write_out.write(writeme)
	else:
		write_out.write(line)
		
read_in.close()
write_out.close()