We are attempting to determine the relationship of block groups to ZCTA. The Census Bureau relationship file zcta_tract_rel_10 shows the percent of each census tract within each ZCTA. We are looking for something similar at the block group to ZCTA level. We are working with ACS data at the block group level and we see that the ACS does reports at the ZCTA level. We assume that ACS is not collecting data at the ZCTA level, so is there a method or information on how ACS is fitting block groups into ZCTA?