It's not about the total, but the prices of individual items as they show up on the checkout screen, which are pretty easy to remember (they're mostly the same week to week).

My local supermarket has some specials which are loyalty card only, so when the item is scanned the full price shows up. I use this to determine when to use the loyalty card for minimum tracking, minimum time, and minimum price (it doesn't have my personal info associated with it, but still connects purchases and it's faster to skip).