I wont go perportional on my racks because they will not handle but 500 watts.... Double ranco will handle ohhh so much more and mine are set to only allow temerature fluctuations of 2 degrees
I have a Herpstat ND proportional and yes it only handles 500 watts but even with 11" flexwatt you can easily run 3 or 4 racks off of it. My racks run 120 watts per rack. The Herpstat holds the temp within .5 degrees and costs under $200