Well, if you only have one person using air it's easy to know how big of a pump you'll need... But often that's not practical... Couple guys running cut-off/die grinders using 4-7 CFM @90 PSI each Maybe an air impact (25+CFM @ PSI but only for a short burst) or an air ratchet (4-6 CFM @ 90 PSI)... then someone running a spray gun or a blast cabinet.. Some HVLP guns use up to 30 CFM, most are around 15-20 CFM... Blast cabinets vary from a minimum around 12 CFM to upwards of 40 CFM @ 90 PSI.. Most everyone I know who owns a blast cabinet that isn't a commercial business waits for their compressor to recover....