That's not performance planning, that's a reasonable technique for confirming the performance you have is sufficient for want you have planned to do.
Hmm,...guess you're right. What can I say, most of my off-airport landings are of the, "hey lets land there" variety.
I'm sure there's some pre-flight performance stuff they taught me, but I'd have to go look for it (think its on my kneeboard), as I haven't actually "planned" to land off-airport since my checkride.