Welcome to the forum!
Yea, you've got the general idea on how to measure sag OK. On street bikes because the shock springs are so stiff, there are truly three numbers you need to acquire in order to get an accurate reading.
The best way to take these measurements are with a few friends and a decent tape measure. Most people use millimeter tape measures, so the numbers I'm working with here will be in MM. Simply hook the end of the tape measure onto the axel in a place it won't move and find a place on the tail fender which is directly parallel to the axel, which doesn't move. Mark it with a grease pencil or something that rubs right off, so you don't forget.
First measurement will be with the rear wheel off the ground. This will be the A measurement.
Second measurement will be with you on the bike and someone holding it. You will pump the shock by sitting on the seat hard. That measurement will be B
Third measurement will be with you on the bike, but don't pump it. In fact, have someone pick up the tail a bit and let it go. That measurement will be C
So the numbers will kinda look like this.
First thing to do is find the average of B and C. In this case it's 332.5.
Next you subtract 332.5 from 360 = 27.5, which isn't a horrible sag number.
Some people talk about static sag, which is the same measuring procedure, but without the rider on board. It's great for determining if the spring rate is right for you. However, rider sag is the critical element and is what your 23mm number is coming from.