First off, it needs to be calibrated. Secondly, the number is generally used as a reference for trending water quality. A single reading is not of much value (at least to me). My ORP drops 50 points every time I feed and recovers over the next 12-24 hours.. I have seen tanks that hover around 400+. Mine stays a little shy of that around 360. If you see upward trends, water quality is improving, down is decreasing. If something dies in the tank, you will likely see significant decline in the short term. Not sure if this helps, but that is how I use it.
BRS and Premium Aquatics sells what you need. Read this http://www.neptunesys.com/PM1-20104031_WEB.pdf and it explains how to calibrate it with 4.0 and 7.0 solutions. It also tells you that you initial readings will be low until it breaks in. I was almost certain that there was a 400mv way to calibrate it, but it has been a while. Now, when looking over some stuff, I see the two step (4.0, 7.0) method is the way neptune does their ORP calibration.