That gives me a 3rd machine with a little over 6100 passmark performance test cpu benchmark score to add to 6300 from my i5 4690 main computer (the main machine actually scores 8400 on all 4 cores but only uses 3 cores to build lighting, so 8400 x 0.75 = 6300) and 4600 from a 3rd machine with a Phenom II B93.
That gives me very nearly 2.7 times as much multi-threaded processing power to build lighting with as with just the main machine alone (again only 6300 since it only uses 3 cores).
6300 + 6100 + +4600 = 17000
17000 / 6300 = 2.698...
IOW, my longest build time (DM-Eliminator with production lighting quality) is about 2.5 hours with just the main computer (6300 cpu benchmark score) working on it and 1.5 hours with both the main computer and the Phenom II working on it (6300 + 4600 = 10900 cpu benchmark score). With the new machine added to the farm bumping the total available processing power to 17000 cpu marks it'll take just under 1 hour (55.6 minutes according to the math) on production lighting quality.
Edit: Rebuilding Eliminator with all 3 machines came out to 57.5 minutes, within 2 minutes of what the math above predicted.
Now just imagine what oh say four OC'd 5960X machines with ~80k cpu benchmarks would do to build times. =)