Its not very scientific, but I usually just make up a 1:9 solution with distilled water from the stock solution. If it looks about the right colour (dark orange/brown) and goes black with starch, then jobs a good'un!
Made a 1:9 solution and it looks right. The other tech is telling me its too dilute but she doesn't know what it should be. Senior tech is off with Covid so I'm in charge. Thanks for your help guys. I could have worked it out but we are rushed off our feet.