You made the right choice by waiting. The TF100 has been upgraded to include a small bottle of the CYA standard solution. That way you can test it and see for yourself what you need to do so you see 50 ppm CYA using the 50 ppm standard. How far to hold it, how to hold it, even, how bright the light has to be. Everything. When you find just the right combination of factors so you get the right reading, you're golden. Then you just repeat that on your pool water and get an accurate CYA reading.