Q: Is it true that there is a known constant ratio between the amount of alcohol in breath and the amount of alcohol in blood? 

A: No.  Although this is an interesting idea and a convenient premise, it is a premise which is simply fictitious.  The ratio between the amount of alcohol in one's blood and the amount of alcohol in one's breath is not a known constant.  In fact, values in scientific literature range from about 1300:1 to 2700:1.  Of course, in order to write a computer program to do the necessary calculations, a particular ratio had to be chosen and incorporated into the program which runs the machine.  The ratio used in Virginia and the US is 2100:1  - meaning our machines are programmed to believe there is the same quantity of alcohol in 2100 parts of breath as 1 part of blood.  Interestingly, the ratio used in Virginia and the US is different than the ratio used in other countries.