Rambam Nezikin Chovel uMazzik 2:11 states anent damages
If he caused him to lose a limb or cut off his hand, he must pay him full compensation for his hand; this being "damages." In this instance, we judge him as if he were a guard at a patch of squash. We evaluate how much such a guard would earn each day and calculate the number of days he will be incapacitated. This is the amount the person who caused the injury must pay.
Similarly, if a person cut off a colleague's legs, we calculate a wage as if he were a door guard. If he blinded him, we calculate a wage as if he worked in a mill. Similar principles apply in all analogous situations.
What's the Rambam's rationale selecting these specific professions, ie limb / hand loss wrt squash patch guard, leg loss wrt door guard and loss of vision wrt mill worker?