How to know when it's time to move to volume 2?

Hi, I practice volume 1 for 4 months and for first 30 days I wasn't doing exercises correctly and then I watched Ken videos repeatedly to get it right .I still
think my chest voice isn't that strong enough .So when I will get to know when to switch to volume 2? I can reach my chest voice upto A4 .how far should I stretch my chest voice?
think my chest voice isn't that strong enough .So when I will get to know when to switch to volume 2? I can reach my chest voice upto A4 .how far should I stretch my chest voice?
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