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.
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.
Sometimes I have to let a to-do item sit for a while before I can be more real with myself about why it's important. FVP still seems like the best way of tackling such things because the structure lends itself to creating mental leverage in easy, but noticeable, ways.
Maybe a small change like writing a number next to each item list representing the number of days it's been on the list would help draw your focus to items that have been lingering a long time. Just start by writing 0 next to a new list item, and adding 1 to the number next to any list item that gets substantially repeated from the previous day. You don't necessarily have to feel bad about list items that have large numbers next to them, but a larger number can create extra weight for prioritizing it.
Another reason this might work is that, much like Space Repetition Systems (SRS) used in reviewing e.g. information on flashcards, there is some logic to dropping cards that you have difficulty incorporating into memory after a certain number of times failing to incorporate it. The logic is that that particular piece of information is either not as relevant to the rest of the set (hard to form associations with "nearby" information), not as relevant to your own interests, or simply too difficult for you at that moment in time. Programs for doing SRS flashcard reviews include code to stop showing those cards again, and you have to manually specify to put them back into the review stack. Basically, it's a sideloop to identify things that actually do require more conscious processing without clogging the rest of the process, which seems to be the main point of FVP.
Combining the two prior points, if you track the number of days on the list, you could have a hard cutoff, like 14, where you take that item off the list altogether. Not sure how to reincorporate it in a streamlined way, though. Having a separate list and maintaining two of them seems onerous.