To get the number for when setting my bike computer I used to mark a line on the ground and start with the valve at the bottom of the wheel over the line. Then get on the bike and roll forward until the valve comes to the bottom again. Then mark the ground and measure the distance in centimetres. Simples!
What will you do if you change your tyres or the pressure, either of which will change the rolling circumference? But then again, it won't change by much unless you make extreme changes. How accurate do you want to be?
I don't remember what mine was from the factory but I do remember that it was incorrect, if you want an accurate value; measure the roll out (circumference) of the wheel which has the sensor on and add that correct value in order to get the correct speed and distance, don't assume GPS is correct, wheel circumference is usually considered more accurate.
For a 27.5" at 2287 sounds a bit too big for me and I would suspect that you won't get the full (true) 25mph/32kph with that number. Bike may stop assisting at 25/32 on your display but you won't be actually doing 25/32.
You will also probably find that your trip kms on your bike are more than your gps recorded kms with that number too.