As with many things in life, what may seem easy may in fact not be. I don't see anything wrong with the logic Hoppy. What type of test kit do you use? My small batch of KH solution was done by trusting my Lamotte KH test kit. I did an initial calculation of how much KH+ I need and then I tweaked the solution by calculating how much RODI water I needed to get to 4 degrees. Then I retested the solution with my test kit to confirm 4 degrees. But, I am sure that is not all that accurate either.

I would much rather just buy some KH reference from Seachem or something if they made it. I don't want to mess with getting all this precision measuring equipment.