On the Kowa, the long eye relief, fixed focus eyepiece is the one to choose/use.
It's about 25 power, or so, and works great at 1,000 yard and everything closer. Will resolve 30 caliber at 300 yards easily.
Resolution is king. Adjustable power is just something to play with.
The adjustable magnification eyepieces have too many lenses. Focus at any power is a compromise at best.