Not sure if I understand your problem, but if your intention is to determine the Biologicl F0 lethality achieved by inactivating Biological indicators (BI), then this is not the way. BI are intended to be inactivated, resulting in either positive (growth) or negative (no growth).
The formula you described is for determination when you have a residual (measurable) bioburden after sterilization or to determine the target F0 you need to have in your process to achieve a given level of sterility (B in your formula).
If this is the case, then, to better understand this formula you have to consider the Spore Logarithmic Reduction (RL or SLR), which represents the tenfold reduction of the bioburden, or the number of log reductions of the population. It fits in the forumla as this:
F0 = D121 * SLR = D121 * (Log A - Log B)
Therefore, “A” would represent the actual bioburden or its worst case value; and “B”, the residual population you want to achieve (in terms of probability, of course, since it is less than 1 CFU).
This “level of non sterility” can be related to the Sterility Assurance Lavel (SAL) or the Probability of a Non Sterile Unit (PNSU) which are not the same thing, but for the sake of calculation, they are considered so. You have to look within the regulation applicable to your process, for the required level of SAL or PNSU, which is commonly required to be not greater than 10^-6 (probability of 1 in a million).