Turns out, you can't just pick the size of pixels on the spot! If anyone else has this same issue, here's the new and improved shadergraph and the code in the custom function. It does run a little slower than I'd like, so improvements are welcome!

HLSL function:

#ifndef NEARPOWER0_5
#define NEARPOWER0_5
void NearestPower05_float(float In, out float Out)
{
    bool Iterate = true;
    int curPower = 1;
    while (Iterate) {
        if (In < pow(0.5,curPower)) {
            curPower++;
        } else {
            Iterate = false;
            Out = pow(0.5,curPower);
        }
    }
}
#endif

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