The LPC user interface is riddled with ways in which to screw up.
Even more so when it comes to non-standard inputs; modified runway lengths or failures.
Whenever I fire up the LPC I always enter the airfield data as soon as I start it and run the perf for MTOW, for 2 reasons. Firstly it gives me situational awareness, it gives me an idea of the speeds, flex and what weight we can lift of the runway in question, and secondly if for some reason I then forget to re-enter correct weight when we get the final then we are covered as we have the most conservative figures possible.
The way I do is not SOP, just a technique, and I know that there is no foolproof way but it works for me.