>- I do not follow 7-20-7 rule and continue zapping for around 40 mins,
This can make you very sick. Continuous requires 1 hour minimum but is nowhere near as effective as 7-20-7-20-7.
My advice is if you can not be serious about zapping, don't do it. Anything done wrongly will often make things worse.
You might consider reading " parasite
Zapping and the Zapper".