Seems that GoogleEarth not have possibilites like GoogleMaps with zooming URL tricks something like "&z=22".
BUT you can use the ALT key in combination with most of keystrokes (keyboard shortcuts) to move more slowly in the indicated direction. That gives you the fine grain control you're asking for:
- holding down the Alt key when using the +/- keys to zoom allows much finer control of the zoom, and hence scale bar values
- or use PgUp and PgDn keys (which I prefer) for zoom in/out.