There are some good threads talking about wr's, and how to use them to reach your goals. I cull everything under 95%. I use a cheap digital scale from the kitchen department of Big Box Store. For me it makes sense not to cull a smaller fish with good growth. It may be my next lunker! I also tag a few fish every year to get some data on movement and indivual growth. It all goes onto a yearly spread sheet.
Good luck transforming the pond into what you know it can be. Have fun and enjoy your "work".
IMHO sage advice CMM is providing here. If you find that you have no fish less than 95%, I would still harvest a few from the size class under your biggest and best to preserve the food supply while keeping the cream of the crop for breeding.
Not a pro...just my 1 cent