My interpretation of the rules ( other interpretations may apply).
Ok
Not ok.
Taking the rules to the extreme, technically ok but may not be by a matter of millimetres. 755m ok, 754.9m not ok. How accurate are your maps/gps etc,its a matter of your own honesty. I try to get as near to the summit as possible but I realise this is not always possible due to geography, weather, etc.
Although not definative the flooding feature on Google Earth is a useful tool to help you work out where the activation zone is and isnt. It can give some interesting results and occassionally I have taken a print of it with me to a summit I am not familiar with to help me get in the activation zone.
I’d also say
I agree with other responses. You are on the right track with the dip at 20m (ok) and 30m (not). The third case is a matter of accuracy that probably none of us have. I think best practice is to be confident you have a metre of height up your sleeve so to speak. So if in doubt, remove the doubt any way you think is sufficient.
The test to apply is “if another activator was here with me, would s/he agree that I’m in the AZ without any doubt?”
We should also not forget that the accuracy of the summit height can often be in question. It can easily be a metre or two out, so as the others have said give yourself a little leigh-way and be sure you’re in the AZ by aiming at a maximum of 22 or 23 vertical metres below the summit using your measurement (GPS/altimeter or map or whatever).
Of course, whenever possible, activate from the very summit if you can. If nothing else the views from there are often better.