Spoke to nipple engagement

I am thinking about ordering replacement spokes for my rd350/mikesxs rim combo. I have the wheels laced with factory spokes just for my amusement. The rear rim seams to have less spoke threads in the nipple than the front. I am thinking about ordering stock spokes from eBay but I don't know how to tell if I have the proper amount of spoke threads engaged into the nipple.

Rule of thumb is usually to have as much thread engaged as the thickness of the rod.

Ok thanks I have much more than the thickness of the spoke.