The amont of difference depends on the cam,as mentioned it will work for some and not others.To claim it's accuracy based on one cam is not a fact for all cams.If it works for you great,but don't claim it will work for all cams.If the "open,close" method works for all cams why insist that the chart is the correct method.A good analagy is if you had a flock of 1000 sheep and 900 grazed on grass and the remaining 100 choose to graze on rocks,which group of sheep would be more healthy and live the longest? There is no point in arguing with the people who use the chart or sheep the eat rocks,it,s like tell your wife who got two black eyes your right and shes wrong,you already told her twice and she still won't listen.