<knit picking on>
When drawing with FM tools a line or other open object can get arrow tips. Their style can be selected. My favorite looks like this

Since long I have notices that the real image of an arrow does not correspond to the properties of the object. Now I have checked this:
This arrow was set up with these properties:
ArrowBaseAngle = 90°
ArrowLength = 786432 (= 0.423 cm)
ArrowScaleFactor = 16384 = 0.25
ArrowScaleHead = 1
ArrowTipAngle = 16°
ArrowType = 3
BorderWidth = 1310720 = 0.706 cm = 20 pt
However, measuring the arrow tip shows an ArrowLength is 2.35cm = 66.7 pt. 120pt (due to a line thickness of 20pt) would be 4.2333cm (what the property claims to be!)
To set up a particular (user defined) tip appearance:
➔ how to set the ArrowLength to really get it 12 pt long (for 1pt line thickness)?
Of couse i could take the approach common to students: introduce a factor sigma (propertional faction to the result of the neighbour)...