surface tension of an aqueous solution

I'm trying to determine the surface tension of an aqueous solution of
glycerin (60 vol% h20, 40 vol% glycerin, @ 20°C) without having to
measure it with a test or expensive device. Is there something similar
to the rule of mixtures for surface tensions of solutions?

pure water @ 20°C = 73 dynes/cm
glycerin @ 20°C= 65 dynes/cm
60/40 vol% = 54/46 wt% for glycerin

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