ZkittleZOG Auto Greenhouse Barney's Farm

Continuing with the Vitax Q4 (7.5mg per plant worked into soil with 1.5ltrs of water - rained most of the week - not sure whether or not this 'delays' things... Still have questions about knowing when to stop feeding and when to start thinking about flushing? Visually it appears that I am 'on schedule' but as this is my first attempt I have to guess ... harvest date is meant to be 70-77 days. I assume it is better to aim for the later date of 77 days? Do autoflowers put on a lot of weight during the last few weeks? (I appreciate that environmental factors determine this), but is that the way they are designed? If anyone has grown this strain outside or in a greenhouse who have a view on the above questions I would appreciate any response.