NEXRAD Latency

I just read this article titled: WEATHER WATCH: DATALINK NEXRAD NUANCES

https://www.aopa.org/news-and-media/all-news/2016/july/pilot/weather-watch

I greatly appreciate this article and thought it was very informative. I do have a few questions regarding the NEXRAD's Timestamp and Latency issues. To help clarify my question I've included two paragraphs from this article followed by my questions.

"One serious issue is image latency. The radar return you see on your screen—be it panel-mounted, on a portable GPS navigator, or iPad presentation—is always out of date. It takes the individual ground-based radars time to scan the sky, more time for datalink service providers to collect the returns from all those radar sites (there are 160 in the United States), and still more time to process them into mosaics. Then, they must be transmitted to you, by either satellites (XM WX) or ground stations (FIS-B). So although your display may report that a radar image is five minutes old, it may actually be many minutes older.

How much older? In June 2012, the NTSB issued Safety Alert SA-017 (amended in December 2015), which asserted that cockpit imagery could be 15 to 20 minutes older than the posted time. The safety alert included two accident summaries illustrating the dangers of image latency. In one accident, a helicopter crew saw an image posted as being one minute old. The Nexrad returns on the cockpit screen showed that severe weather was seven miles from the destination airport. In truth, at the moment it was posted on the display the image was five minutes old, and the weather was in fact crossing over the destination airport."
 
My questions are: How can you tell the true age of the Radar image, if you apply the above statement from NTSB that "cockpit imagery could be 15 to 20 minutes older than the posted time."? I use ForeFlight and if you read the timestamp in the top left corner of ForeFlight Maps page, to obtain the timestamp of the image, it's usually 5 minutes to 8 minutes old. Reading this article it appears you definitely have to add some additional time to this timestamp in order to obtain the true age of the radar image, but the question is how much extra time do you need to add to the timestamp? Is there some kind of rule to apply to this? Does the time you add to the timestamp vary depending on factors? Which factors? It appears from this article that the image may be a few minutes up to 20 minutes older than the timestamp. This is ambiguous and complicated not knowing how much time to add to the timestamp, since the NTSB is stating that the the image may be 15 to 20 minutes older than the timestamp and the article states it " may actually be many minutes older " than the timestamp. Can you please clarify this question for me.  I would really like to know how much extra time I need to add to the timestamp in order to know the true time/age of the radar image. 

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