Speaking as someone who hit "Edit" on his post over 10 times before checking the FAQ: if you haven't messed with your profile settings about handling comments/posts yet, save yourself some time and just check the FAQ before trying to add spoiler text. The right formatting wasn't as obvious as I expected, although it was simple.
Starting assumptions: impact is measured on a per-belief basis, depends on scale, and is a relative measurement to prior expectation. (This is how I am interpreting the three reminders at the end of the post.)
To me, this sounds like a percent difference. The change between the new value observed and the old value expected (whether based in actual experience or imagined, i.e. accounting for some personal bias) is measured, then divided by the original quantity as a comparison to determine the magnitude of the difference relative to the original expectation.
My sentence: You can tell that something is a big deal to you by how surprising it feels.