If you're really having a problem with the ballast rolling away, you can use a 3/4" brush to "paint" full strength white glue on the shoulders of the cork, then apply the ballast, arrange it with a clean, soft 3/4" brush, then spray with wet water followed by an application of diluted white glue. The "wet" water will draw some of the glue up from the cork into the ballast, making for a complete bond. Generally, it's much faster (and cheaper, too) to ballast the entire track at one time, and the same goes for areas where there are multiple tracks. The diluted white glue, after application of the wet water, will travel readily from the area between the rails to the area outside of them, requiring, over-all, less glue and less time spent. Essentially, you're ballasting each area twice.
Personally, unless the "ground" really drops away at the side of the tracks, I prefer to let it find its own slope; after all, a container of ballast goes a long way, and is not really
that expensive.
In the picture below, the diesels rounding the curve at Hoffentoth Bros.' yard in South Cayuga are on a fill. I painted the slope below the tracks with white glue, then dumped rip-rap (broken plaster) alongside the track. This was followed by some coarse ballast, then a
lot of cinder ballast. Finally, the regular ballast was applied. Each successive application of material was allowed to find its own slope, then the entire scene was soaked with "wet" water and diluted white glue was then added. The unfinished field at the bottom of the fill became a lake of diluted glue, and the whole area took over a week to dry, and even longer to fully solidify.
Wayne