About code problem

[GithubRyze]

Dear friend:
long time ago, I was download this project for study how to draw view! but i do't understand you calculate the Thumb position at all !
int arcStart = (int)mProgressSweep + mStartAngle + mRotation + 90;
mThumbXPos = (int) (mArcRadius * Math.cos(Math.toRadians(arcStart)));
mThumbYPos = (int) (mArcRadius * Math.sin(Math.toRadians(arcStart)));
and in onDraw function:
canvas.translate.(mTranslateX - mThumbXPos,mTranslateY - mThumbYPos);

why arcStart value need add mRotation and 90 ???? if you viewed my question please tell me? thank very much!! and it is very nice to solved the diff quadrant problem!!