How to resize bitmap image to be <200 KB and meet Tile restrictions (WinRT) - windows-8

Question

I am developing a routine to scale some bitmap images to be part of tile notifications for my Window-8 app
The tile images must be <200KB and less than 1024x1024 px in dimension. I am able to use a scaling routine to resize the source image as necessary to fit the 1024x1024 px dimension limitation.
How can I alter the source image to guarantee the size restriction will be met?
My first attempt was to continue to scale down the image until it clears the size threshold, and use isTooBig = destFileStream.Size > MaxBytes to determine the size. But, the code below results in an infinite loop. How can I reliably measure the size of the destination file?
bool isTooBig = true;
int count = 0;
while (isTooBig)
{
// create a stream from the file and decode the image
using (var sourceFileStream = await sourceFile.OpenAsync(Windows.Storage.FileAccessMode.Read))
using (var destFileStream = await destFile.OpenAsync(FileAccessMode.ReadWrite))
{
BitmapDecoder decoder = await BitmapDecoder.CreateAsync(sourceFileStream);
BitmapEncoder enc = await BitmapEncoder.CreateForTranscodingAsync(destFileStream, decoder);
double h = decoder.OrientedPixelHeight;
double w = decoder.OrientedPixelWidth;
if (h > baselinesize || w > baselinesize)
{
uint scaledHeight, scaledWidth;
if (h >= w)
{
scaledHeight = (uint)baselinesize;
scaledWidth = (uint)((double)baselinesize * (w / h));
}
else
{
scaledWidth = (uint)baselinesize;
scaledHeight = (uint)((double)baselinesize * (h / w));
}
//Scale the bitmap to fit
enc.BitmapTransform.ScaledHeight = scaledHeight;
enc.BitmapTransform.ScaledWidth = scaledWidth;
}
// write out to the stream
await enc.FlushAsync();
await destFileStream.FlushAsync();
isTooBig = destFileStream.Size > MaxBytes;
baselinesize *= .90d * ((double)MaxBytes / (double)destFileStream.Size);
}
}

Answer 1

Can you not calculate it using width x height x colourDepth (where colourDepth is in bytes, so 32bit=4bytes). Presumably you're maintaining aspect ratio so you just need to scale down width/height until you find it less than 200KB.
This assumes the output is an a bitmap and therefore uncompressed.

Answer 2

Considering that tile size either 150x150 for square tiles or 310x150 for wide tiles you should be able to shrink image down to the appropriate size and with jpeg compression you are pretty much guaranteed to be under 200k. Set compression quality around 80. It will give you good compression ratio while keeping decent image quality.