question

The following user command accepts the conveyor, flowunit name and desired max flow rate and looks at the width settings of the conveyor in order to calculate and set the speed.

Until the mass flow calculator is fixed to account for the width value and rules you should probably use this if you're not using the physical width:

//setSpeedFromMaxFlowRate(<conveyor>,<flow unit name>,<max flow rate>)
Object conveyor=param(1);
string funame=param(2);  //flow unit name
double maxrate=param(3);

treenode flowunit=Model.find("ConveyorSystem>variables/flowUnits/"+funame);
if (!flowunit) {
   mpt("Flow unit "+funame+" not found in the conveyor system definition");mpr();
return 0;
}

double Ux = flowunit.subnodes["length"].value;
double Uy = flowunit.subnodes["width"].value;
int isRound = flowunit.subnodes["isRound"].value;
double R = isRound ? 2.0 / sqrt(3.0) : 1.0;
double maxDensity = R / (Ux * Uy);

double width=conveyor.getProperty("Width");

string rule=conveyor.getProperty("WidthRule",GET_PROPERTY_FLAG_STRING );
double widthval=conveyor.getProperty("WidthValue");
double effectiveWidth=width;  //rule ="Conveyor Width"
if (rule=="Number of Lanes")
  effectiveWidth=widthval*Uy/R;
else if (rule=="Virtual Width")
    effectiveWidth=widthval;
else if (rule=="Single Lane Flow Unit Distance")
    effectiveWidth=Ux/widthval*Uy/R;  
else if (rule=="Single Lane Gap")
    effectiveWidth=Ux/(Ux+widthval)*Uy/R;   
else if (rule=="Percent of Upstream Width") {
    mpt("Percent of Upstream Width - unknown effective width");mpr();  // unknown
return 0;
}

double speed=maxrate/effectiveWidth/maxDensity;
conveyor.setProperty("Speed",speed);

I've not found the definition for "Percent of Upstream Width" - whether that just means physical size or the effective width of the upstream conveyor (and what if there are more than one with different width rules?).