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Ray, I'm happy the curve resembles a V however I'm not too convinced the slope of the LH vs RH side of the V is correct. If I fit regression lines to the data either side of focus I'm getting quite different slopes if I include all the points. If I restrict the data to points close to focus but not in the flat part of the curve I get much closer gradients. I'm not sure what SGP is doing behind the scenes with the data but if it is simply the intersection of the two lines then the different slopes impacts the focus position. I've tried to demonstrate this in the attached image which shows two sets of regression lines - one using all the data points and the other selected ones.
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