You'll get your best, most consistent results, if you thin Tamiya acrylics.  As I described above, you could either mix a little of the color and the X20 thinner, in the well of an impermeable palette, or, use a little jar of the thinner, dip your brush in the color, then in the thinner, and let it blend on the target.

While I was learning to use Tamiya paints with my airbrush, I read somewhere that they are formulated for airbrushing and are intended to be thinned.  I don't know if that is true or not, but I do know that that inspired me to thin the paints for handbrushing-I already thinned them for airbrushing-and that's when I got the best results, consistently.

For me, neither water nor isopropyl worked to thin Tamiya acrylics for handbrushing. That's why I recommend using Tamiya's proprietary thinner for handbrushing.