The reason for diluting to 65% is to lower the vapour pressure.  The higher 
concentration was found to flash to quickly and result in queen loss.  Some 
even go down to 50% (Amrine).

The calculation is quite simple, since, unlike mixing some acids with water, 
the volumes are for all intents and purposes additive and there is no 
exothermic reaction to fear.  It is like adding water to water in that 
regard.

In other regards, handing formic requires knowledge and respect and 
protective equipment is advised: eye protection at minimum.

I cover this in detail under selected topics at www.honeybeeworld.com and 
won't go into it here, especially since the archives are full to overflowing 
with information (and the usual uninformed and fearful speculation)... 

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