The FBP is defined by the ASTM F-316-03 standard  as the pressure at which the first continuous gas bubbles are detected. In practice FBP is associated as the largest or maximum pore size.
The calculation of the FBP requires to select a certain minimum flow (e.g. 30, 50, 100 ml/min) and when it is achieved record the pressure. Then this pressure is used to calculate the FBP size. The question is to select the minimum flow through the sample and the major disadvantage is that this minimum flow is different for every sample and it is not easy to determine a priori. If a certain minimum flow is selected for the calculation it is possible that the largest pore in the sample was already open for a while before that particular flow was determined.
With the step/stability method it is possible to measure the true FBP. When applying a constant gas flow, before the opening of the largest flow the pressure increases in a linear way. At the moment that the gas flow passes through the sample via the largest pore the pressure increase drops and it is this particular pressure the one that corresponds to the FBP of the sample.