You have to set the reserve level for the mineral in question (and all others I think) to the value you want then you can just do a load command and it will load all minerals but leave behind the reserve amount.
There is a problem I think with the way the command is impllemented that means give the command, let if finish and then manual remove it before giving any further commands else with the ship/task force seems to bork up and get stuck in a status of still trying to load even though the ship has actually loaded and the ship may be on the way to another destination. This may be because there is suddenly a tiny amount of minearls over the reserve level or something.
I am also not sure it works with a reserve level of 0, but I have never tested that specifically.