How much salt would you recommend to start out with for this roasted beef stock?

I would not add any salt to start with, honestly. This is stock, which gets confused with broth - and unfortunately both terms seem to get used interchangeably. But they are not the same things. Stock is an ingredient, broth is a sort of soup. You could use stock to make a soup, but you wouldn't want to drink it on it's own. Since it's meant to be an ingredient to start off other things, you don't want it to be seasoned yet- because you don't know where you might be adding it in. It's used instead of plain water in all sorts of things, from soups to pilafs, sauces, and such- and what is a good seasoning for an Italian polenta is not a great idea for a tagine, for instance. So although it's tempting to add in seasonings and salt, don't do it. The only possible exception to this rule might be if you do a stock using leftover bones or bits from a dinner. It won't be much - and you will know what it is bringing so can make decisions for use based on that. But this is an example of a pure, purpose made stock- and is meant to be bland in terms of seasonings.