Doesn't the manufacturer supply Isc numbers? Why wouldn't you just use them?
They do. However, because of back surface irradiance, the Isc values will be greater than when STC applies only on the front surface. Design with ordinary modules doesn't require you to think about this. The manufacturers commonly give you test data of Voc, Vmp, Imp, and Isc with 10% backlighting, 20%, etc, all on the assumption that 1 standard sun of irradiance is available on the front side. The OP's question is how to know which one of the backlighting test data tables should be used, when sizing everything else connected to the bifacial module? I would design by the highest backlighting that the manufacturer tests, for a worst-case scenario.
I'd like to see examples of typical backdrop surfaces that are known to produce each percentage of backlighting. Such as white TPO roof produces X% backlight, parking lot with cars below produces Y%, green landscaping is Z%, gravel/stone/concrete is XX%, etc.