If you really need to save on reagents here - and sometimes there are legit reasons to do so, e.g you're in the middle of a SLAM and are running out of reagent and have to survive until new supplies arrive - then I'd suggest the following.
Don't just use a 5ml sample. This not only reduces the resolution to 1ppm per drop, but you will also introduce a bigger influence of getting the 5ml not quite right (compared to not getting 10ml right), and you meddle with the intended sample to drop ratio that the test is designed for.
Instead, use a larger sample and dilute that 50:50 with distilled water, mix well. Then take a 10ml sample of the diluted water, 1 drop of reagent will be 1ppm.
I think, like that the testing errors will be a little smaller compared to just taking a 5ml sample.
I wouldn't recommend that for standard testing, but I think it would be OK to keep a SLAM going until new supplies arrive. Especially if you then also increased your SLAM level by about 1ppm to make room for higher testing errors. So you'd basically pay with chlorine to save on reagent.