Graphic properties of arrows

<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)...