The reading time is estimated by word count, so you could just check in Medium for reference and then just run the word count function in Word if you still want to use that. I think five minutes is around 1200 to 1500 words. People read at different speeds and how you write, particularly whether you use short sentences and paragraphs, can speed up the actual reading time, but not the estimate obviously.

Another good thing to do is read through the draft on your phone so you get a sense of how it looks on the mobile reader. Then change the graphics or break up paragraphs accordingly.