Hi RunSick. The basic problem is that theoretical performance in the LPC is only mechanised for two scenarios - no thrust reversers working at all or both thrust reversers working and generating max reverse. The scenario you discussed falls outside these two cases and it therefore rejects it as an invalid computation. Clearly it is unsatisfactory, but once you understand this limitation you just feed in the case which most clearly matches your situation (ie the one which gives the worst result to create a safety margin). I am not saying it is good, but it is vital to understand the system to make it work in the most safe sense for you. I hope that helps.