i was looking over ghosting's post and could follow the last part but just couldn't figure out the where the 2.1 was coming from. when i tried the calc i used 1g or spiro equals 1ml of solution(pg,iso,glycerin). as sir kevin says 5% of 240 would be 12ml or 12g. it should actually be a little less than that since this only holds true for water(1ml of water has a mass of 1g). formula weight is sometime used for concentration calcs. the weight of spiro is 23x that of water. water is 18 and spiro is 416. wouldn't it be possible to say that only 1/23g of spiro=1mlor 1g=23ml. that would be approx 43mg=1ml. at 43*12=516 mg would be necessary to displace 12ml of solution. this is only an idea. does it sound possible?