40 is good actually. Like mine was 80, but the difference in boldness in color from 40 to 80 was weak, so it didn't help much in terms of reading.
To get proper test results, you can do the following:
- Fill Cup 1 with 200 mL of distilled water, as well as Cup 2 with 200 mL (200 mL in each of those 2 cups)
- Put 0.8 g of SN in Cup 1 with 200 mL
- Remove 2 mL from Cup 2 to reduce from 200 mL to 198 mL
- Extract 2 mL from Cup 1 and add it to Cup 2 which now has from 198 mL back to 200 mL
- Use Test Kit Strips on Cup 2
Essentially, Cup 1 has 800 mg of SN / 200 mL of Water, when you remove 1% from 200 mL to 198 mL from Cup 2, and then add 1% of the 800 mg of SN, you come out with 8 mg of SN / 200 mL, hence:
8 mg / 200 mL -> 40 mg / 1 L
The 40 mg is in reflection to the max 40 ig you have.