[Stk] calculating a delay in physical model

[Michał Junczyk]

Hello everyone,
I am new to this list and STK as well. STK and "RSS" book are my
starting point into synthesis algorithms. Thanks for providing tools
which help clarify understading at the code level.
I would like to ask about a calculation of delay line length. From
theory of acoustics I assume that for open-closed pipes delay length
should be equal to:
(samp.freq / freq) * 0.5
However I've noticed that it is decreased by small value, depending on
the model e.g. in a clarinet model delay is calculated as
(samp.freq / freq) * 0.5  - 1.5 and in sax: (samp.freq / freq) * 0.5
- 3.0 and flute: samp.freq / freq - 2.0
Does it depend on the length of the loop filter? Right now I have
correct results in my matlab code derived from clarinet class, but I
am just willing to understand it. How I should properly calculate
length of the delayline and delay in my own program?
I expect that's a newbie question... Thanks in advance for an answer.
MJ