With manual drains I am having a hard time pin pointing the reason for the 75% spec. As mentioned by others 75% is not an unusual RH for fla and my location in SE texas. Most compressors I have been involved with are impossible to pump water adn cause damage. they have reed valves so you can not push water to a dead head. there fore any moisture would be pushed to the next mechanical separator. the mext thing is that 75% at 90F is no longer 95% whenthe gas igets heated to say 130F from compression. Probably the reason why there is no drain capability for he first stage output.
I was looking for documentation o the compressor and found a picture which shows only one moisture separator on the final stage. I am wondering whether the 75 spec is set to provide a life measurement of the filter. IE 40k cuft on a filter so long as the compressor does not exceed 75%. which is not a factor of the cpmpressor but the filter system used in the package. You have what looks like a 4 cuft compressor at 225-330 bar output. 75% for the system may be valid but not a concern imo.