Having read this thread, I decided to check my ref solution against a Nutrafin KH test kit. I did the check using 2 x the volume of water (solution) and 2 x the number of drops.

The test kit says my solution is 3.36dKH. I'm fairly happy with that, given that there will be inaccuracies with test kits.

I made the reference solution up according to Toms suggestion of cooking off the soda and mixing up an initial 40dKH solution then diluting that down.
The solution was made about 6 weeks ago and has been stored in an airtight bottle since...

I refilled my drop checker about 2 days ago and it still seems to be working fine.