PictureBoxの中で余白を考慮して計算
double imgW = picArea.Image.Width;
double imgH = picArea.Image.Height;
double picBoxW = picArea.Width;
double picBoxH = picArea.Height;
double magW = picBoxW / imgW;
double magH = picBoxH / imgH;
double zoomRatio;
double hMargin;
double wMargin;
if (magH < magW)
{
zoomRatio = magH;
hMargin = 0;
wMargin = (picBoxW - (imgW * zoomRatio)) / 2;
}
else
{
zoomRatio = magW;
hMargin = (picBoxH - (imgH * zoomRatio)) / 2;
wMargin = 0;
}