I can't answer your question directly but offer the following to consider. First, the propylene glycol you used is not intended to be diluted as it appears you saw. Unless it is undiluted, it's freezing properties are essentially the same as water. It is therefore important to ensure the glycol displaces all the water.
Second, the answer you are looking for is dependent on the size of your muffler, assuming you have one and it is a water lift type which also contains water that can freeze and burst. For this reason, it is also important you make sure you have displaced water in both the heat exchanger and muffler and as we can't know the type and size muffler you have, the precise volume of antifreeze cannot be reliably known until you see it coming out the exhaust.