First off, I would make sure to have the right quantity and correct type of bones. Veal shank bones contain more gelatine than similar shanks from full grown cows. By weight, I would suggest 10kg nicely roasted veal bones to 20kg COLD water and 5kg assorted mire poix, a nice herb sachet and some tomato paste (I caramelize my tomato paste in equal parts duck fat until well browned) and bring to a lazy, key word LAZY simmer, very minimal amount of bubbles coming to the surface. Make sure to skim every hour or so and overnight you will have a nice veal stock with a nice color, body (which you want when you try to reduce) and if it has simmered slowly, nice clarity. However, if you find that you stock is missing body, oxtail will provide body to spare. A little will go a long way. Sorry for the long post, great sauces start from great stocks!