There is no true 'preform temperature' because there is dynamic range of temperatures along the entire length of the preform.
Question: What should the finish temp be on the preform as it leaves the oven?
My Response: There is no specific temperature, but typically a profile this is specific to the material, mold, and equipment you are processing. For this reason, there may be an average temperature recommendation such as 100°C which might be used, but it can be very misleading. There is a range of temperatures which typically vary between 80-110° along the length of the preform. One of the most important aspects is to ensure you are measuring different locations on the preform at the exact same location each time. A hand-held probe will not give you repeatable, reliable, or useful results.