If you thin your highly developed white ink, it will be like milk that will soak and penetrate the shirt. You won't be happy with the result. You are probably trying to change the ink because it is the only thing you can change, once you made that screen. I don't think it's a good idea to ruin the opacity of your white ink, focus on what you can do, to make the ink transfer through the mesh and form a film on the surface of your shirt.
The single most important variable for printing white ink is higher tension, or the crutch of raising the off-contact, so the squeegee increases the mesh tension as it brings itself into momentary contact with the shirt.
Remember this will also stretch your image as you stretch your mesh. Not good for registration, but your image will print better. Be Fred Astaire, not The Hulk.
- High tension
- Low off-contact
- Stiff blade
- The ink has no chance!
Make a test on scrap fabric and raise the off-contact as high as you can. Put wooden stir sticks under the frame to elevate it. Print the image with a stiff blade so it doesn't bend as it has to overcome the mesh tension. If you have a putty knife, print an image with that too. Yes, you might have to round the corners of the blade.
Make another type of test by printing your image on a piece of dark paper. It will be perfect. Cure the paper. Peel the ink off the paper. Spread it on a dark shirt.
That is what your ink and screen are capable of.
This is why the transfer is still the best white ink image - all on the surface. We can learn from that and try to duplicate the idea.
thining plastisol 
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