Comparing price per 100g and kg is easy, just move the decimal place one.
Yeah, I don’t mind that too much. Amazon uses per KG and Walmart tends to use per 100g on things like cereal, so it’s an easy conversion.
It does help to shop at the same store(s), since you kind of know what things should be, even before unit prices come into play. That is, assuming you aren’t caught off by shrinkflation, then you need to also remember the size/weight of an item to know if it’s a good deal based on price alone.
This is why 99% of my grocery shopping is done online, building a list first, rather than on-the-fly inside of store. Once I’ve done my comparison shopping and built a list, I go to shop or set aside the order for pick-up.