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Old 10-03-2010, 07:19 PM
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Garyh
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Join Date: Jun 2006
Location: Glen Innes, N.S.W.
Posts: 2,888
Don`t know the formula etc but have you looked at Mel Bartels site for calculating diagnal size?
Gives you the illumination off axis ..
Hope that helps a little.

http://www.bbastrodesigns.com/diagonal.htm
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