To VB Guru or experts....

Here what I was going to do is working on a project based on my finals. what I intend to do is to get getting needle to drawn on picture box... and at the point it will change based on values at run time. But at this point I am unable to understand how it work or what these calculations defins so I am kinda hoping anyone with knowledge or experience in trig calcuation would be able to explain it to me clearly... that might help me out to do a few adjustment on calcuations. so here is the codes:

Code:
 if PSI < 60  then   

  PSIANGLE  = 0-((60-PSI)*0.75)

Else

  PSIANGLE = ((PSI-80)*0.75)

End if
My understanding...
Code:
if PSI < 60  then
It states if the Value 'PSI' Is less than 60

what I dont understand is....
Code:
  PSIANGLE  = 0-((60-PSI)*0.75)
or
PSIANGLE = ((PSI-80)*0.75)

0.75 is a twip... actually what is twip, what does it have something to do with it? Also the calculation on
(0-))PSI - 60 ) *0.75( and ((PSI - 60 )* 0.75 seem to vague to me

Thats a code for needle to display into pictrue box

Code:
PICPSI.Line Step(0-(500 * Cos (PSIANGLE)), (-0-(500 *Sin(PSIANGLE)))-Step(0-(250 *Cos (PSIANGLE)),(0-(250*Sin(PSIANGLE)))
I understand this is making a lines 'needle' to center... but if I was going to try to do few adjustment, I would need to understand how to do this

FApparently, it sound complicated.. If anyone couldnt get toe xplain this would you happen to know which website that provide information or clear expaination on these code definations? Let me know

Thanks ina dvance