evaluate curve automatically reparameterized

hey,

i have troubles some troubles with closed polycurves. i want to offset each curve to the inside but i will get bad results as soon as some curves overlap. so i decided to draw a line from each corner to the center of the polycurve area and now i want to draw a point with 0.5 along the line. anyhow the lines are all reparameterized and 0.5 will be the midpoint in each line. can someone tell me why this is? or does somebody know a way to a clean inner offset either with scripting or gh components.

thx!