It sounds like you have a hvac tech that know what to do. Remember him when you have any future problems.
In case you're curious... Fixed flow refrigerant metering devices (which you likely have, as opposed to a thermostatically controlled valve (TXV)) depend on a certain minimum heat load in order to be able to vaporize the liquid refrigerant.
Reducing the air flow changes the effective heat load being brought to the evaporator coil. It may dip to below a load value that the metering device was designed around. The refrigerant may not get vaporized completely before leaving the evaporator coil. (it's called an evaporator coil for a reason) That may make adjustment necessary and more critical than normal in order to avoid flooding liquid refrigerant back to the compressor and possibly liquid slugging it. That can damage a compressor since liquid is very hard to compress and it could also wash the lubricant from critical moving parts.
Now that you know that, you may want to look at the compressor dome after the unit has been running in the low fan mode for an hour to make sure it isn't sweating, which would indicate that even more refrigerant may need to be withdrawn. It's common for the suction line (larger tubing on the compressor) to be cold and even sweat but the compressor body never should.
Sometimes a tech is not able to spend the time to see if his adjustment is perfect, so he'll not mind if you give him a little help or even a call to say he did it perfectly. I often told customers that they can get involved and save both of us some time and money... most liked that concept... especially the money part.