So I recently got a $15 split screen off ebay from some HK seller.
Since it is just $15, I have nothing to lose, so I gave it a try. I am using K-x body, can not guarantee on other bodies. (jump to conclusion if you don't want to read the situation)
The first time I install it, it is CRAZILY front focused when I rely on the screen.
The green hexagon dot works fine, I still get perfect focus shots relying on the dot.
After researching, I find out about the shim (the little "C" or "U" shape metal thing in between the screen and body).
I took the shim off, it is a little bit better but still front focus a lot.
There is no way to reduce the thickness of the screen, so I FLIPPED the screen over.
And now it is back focus a little bit.
So I put the shim back in place (body-"shim"-focus screen-metal cover, don't put it in between the metal cover and the screen, it doesn't do any help)
Now it is working nearly perfect (still have a very tiny front focus, guess that's just the shim is a little bit too thick)
Now I can make 80% in focus shot with ONLY the split screen and 90% in focus shots with split screen+green hexagon indication
Conclusion:
if your situation is:
front/back focus image result when rely on the split screen, but green hexagon dot works fine.
if your screen is front focus, take the shim off;
if your screen is still front focus after taking shim off, just flip the screen over(ignore the tab on the screen)
if your screen is back focus, put the shim on;
if your screen is still back focus, you need thicker shim;
if your screen is front focus a little bit after using the shim, you need thinner shim.
Review on $15 Ebay split screen:
Haven't use the more expensive katz-eye or focusingscreen.com
But the cheap ebay one is giving me decent result and usage.
Sure it is a little bit darker than the default screen & may be dark than the more expensive katz-eye, but for this price, it is just picking bones from egg.
Also the good thing is if you are first time changing your focus screen and you accidentally scratch it or dusted it, you won't feel the pain as on a katz-eye.
In general I am very happy with my cheap ebay screen and I do recommend anyone who's thinking a split screen to try it.