Well...first you don't need to do 5-day treatment and then continue another 5-day treatment again 10 days later. That is TOO much.
I doubt you poisoned him but it's possible you squirted it down the wrong pipe which could be a problem.
Can you take a short video of him right now and upload it...
Sucking up the med I to a syringe and dosing each one to the beak is the best way to ensure they each get the correct amount.
Under dosing is just the same as not even dosing them to begin with. It's actually worse, it will allow the worms to build up a resistance to the med.
Adding it to...
The correct dose amount is .23 ml per each pound. So if you only gave her .6ml you under dosed her if she is 6 pounds. You did not give her too much.
She most likely had something else going on. I am sorry for your loss.