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http://en.wikipedia.org/wiki/Diastatic_power I found this information:

A malt with enough power to self-convert has a diastatic power near 35 Ã‚Â°Lintner

Is it too simple an approach just to calculate a weighted average of the diastatic power and check if the number is greater then 35?

An example:

Pale Malt: 2,1 kg * 100 Ã‚Â°Lintner = 210 kg*Ã‚Â°Lintner

Munich II: 1,9 kg * 25 Ã‚Â°Lintner = 47,5 kg*Ã‚Â°Lintner

Carafa I: 0,7 kg * 0 Ã‚Â°Lintner = 0 kg*Ã‚Â°Lintner

Special B: 0,70 kg * 120 Ã‚Â°Lintner = 84 kg*Ã‚Â°Lintner

Caramel/Crystal Malt - 60L: 0,60 kg * 0 Ã‚Â°Lintner = 0 kg*Ã‚Â°Lintner

Weighted average = (210+47,5+0+84+0)/(2,1+1,9+0,7+0,7+0,6) = 57 Ã‚Â°Lintner > 35 Ã‚Â°Lintner, i.e. sufficient diastatic power.

By the way: In BeerSmith, the diastatic power of Special B is set to 120 Ã‚Â°Lintner. That seems way to high for such a dark malt. Is 120 Ã‚Â°Lintner the correct value for Special B?