Now let's say the instead of pointing straight up, the needle object is pointing to the left, 90 degrees to the left of vertical.
We now need to add in 90 degrees just to get the needle to point at 0 on the Altimeter gauge. The new equations become the following:
if (altitude < 100) angle = rad ( 90 + altitude*225/100 );
else angle = rad ( 90 + 225 + (altitude -100)*45/100 );
Clear as mud?