Re: A 0.1 ml of both? can we use "a"?
If you want to use "A", you'd have to say "A tenth of centilitre of ..." or "A hundredth of a litre of ...". As it stands, the article is incorrect. In general, the rule about not starting a sentence with a numeral stands, but in this case it would be fine. It would be read as "Zero point one millilitres of broth ...". I wouldn't necessarily expect the rules of written English to be too important in what appears to be some kind of scientific equation or calculation.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.