The 10mL test will have a +\- 0.5ppm error (Taylor error is basically +\- 1 drop)

However, you had an OCL (overnight chlorine loss) of 2.5ppm. Therefore, proper procedure is to immediately readjust your pool water back up to shock level and hold it there all day. If no one is able to test and adjust during the day, then you could go over the shock level by a few ppm's and then check again in the evening.

Basically you need to maintain shock chlorine levels until your OCL is 1ppm or less. I know it sounds tedious, and it is. Typically, the algae kill rate is high in the beginning and then the process tails off towards the end as your dealing with organic oxidation. It sometimes helps to backwash or clean the filter during this process to remove a lot of the dead algae (think of it as algae corpses rotting in your filter medium and emitting lots of organics into your water). Filter backwashing speeds up the tail end of the process which is the oxidation of organic compounds into volatile chloramines.