Like Kalleheikki said, view both mask and image simultaneously by turning on both visibility eyeballs in the Channel palette (same function as backslash) and just paint straight in the mask layer. If the rubylith is obscuring your image too much, just double-click the mask layer in the Channels palette and set the opacity down. You can change the color of the rubylith there as well.
I’m thinking your procedure might be flawed. Make your selection in your image first, then click the “Add layer mask” button in the Layers palette. That will give you your base mask in the shape of your selection. If it’s opposite what you want, just Invert. Then paint in the mask with black (to hide) or white (to reveal) anything you missed with your selection. And with that image, I’d make my selection with the pen tool for cleaner edges.
It is easier to destroy than to create.
More fun, too.