Do you mean a 40 inch vertical? Unless we're talking some verrrrrrrrry short arms, anybody at your or Xen's height would be able to do way more than grab rim with that type of vertical.
Spud Webb was only 5'7, and his 47" vertical was out of this world, and won him an NBA dunking title.
Anyway, a very easy way to test it is to find a tall, flat wall, wrap some tape inside-out around your finger tips, and reach as high as you can and put it on the wall.
Then, repeat the process, but jump up as high as you can and slap the tape on onto the wall. Grab a ladder, and measure the distance between them: that's your vertical, more or less...