The 63-66° and 66-69° exist because there are two types of bicycles.
Mountain bikes have small diameter 559mm wheels and a high bottom bracket for a lot of ground clearance. The combined effect of the small wheel and the required high ground clearance mean the chainstays are nearly horizontal. Thus, the angle between the seat tube and the chainstays is rather high, about 66-69°.
In contrast, road and hybrid bikes have large diameter 622mm wheels and the bottom bracket is typically put as low as possible for making it easy to stop and making the ride feel of the bicycle stable on roads. Thus, the chainstays angle downwards towards the bottom bracket, meaning the angle between the seat tube and the chainstays is low, about 63-66°.
Your Kona Dew is a hybrid bike with 622mm wheels. Most likely you measured correctly: the angle between the seat tube and the chainstays should fall between 63-66°.
Of course you can calculate it based on seat tube angle, bottom bracket drop or height and chainstay length. Your seat tube is at 72.5° angle. Chainstays are 440mm. Bottom bracket height is 280mm. The rear hub height is about 347mm for those 622-37 tires. Thus the rear hub is 67 mm higher than bottom bracket. asin(67/440)*180°/pi = 8.8°.
That should give 72.5° - 8.8° = 63.7° which falls between the angles 63-66°. So there was quite large uncertainty in your measurement indeed.