How do I get a Hill slope from saturation binding data?

Adapt the saturation binding equation to fit a slope factor. The usual saturation equation is

Y = Bmax * X / (Kd + X)

where X is concentration and Y is specific binding. To fit a model with a Hill slope, fit instead to

Y = Bmax * X^h / (Kd^h + X^h)

Where h is the hill slope. You'd need to enter this as a user-defined equation. For initial values, set

Bmax = 1 * Ymax Kd = 0.1 * Xmid h = 1

You might find better rules, but those are a starting place.