You have a great amount of room to use fiat here. The random rolls are a good way of handling background situations.

My understanding of stock markets is novice, but there are some fundamentals to keep in mind.

Extraordinary circumstances have the ability to massively alter a stocks valuation. For example Nintendo's stock doubled after Pokemon go was released despite them having nothing to do with game and recieve very little of the game's revenue.

Your players are likely to be those extraordinary circumstances. If you look at Apple's stock around the time of their new product release you'll see a very predictable pattern of the stock price increasing months before the release and a wound down after. This has a lot to do with people trying to bank on Apple coming up with a product that will take off.

So stocks react more to word on the street than actual numbers. Earnings reports and a companies assets are the usual basis that long term investors look at in a company, but it's also the market they exist in, the likelihood of them gaining marketshare, how lean their operation is. Major investment companies set the tone and their consistent investment is often the water mark that a company tends to hold at.

So applying this in a rpg, you'd want your company's stock to make small leaps when the players manage to get new revenue streams going, get a new major client or have a new innovation about to reach the market. Not certain of what kind of company your PC's are running but if they are part of research or sales I would have NPCs mention to them that the companies stock climbed after such and such article was written. I would make the companies stock plainly available. It could be funny to adjust the stock negatively if the PCs goof something up.