First, rather than ceiling bounce, I'd suggest bouncing the flash off a large piece of white foam core placed to the side of the subject. That way, it will look more like window light, and thus more natural.
There are a variety of ways to approach the problem of determining exposure. But, the most feasible approach depends on the specific features of the flash you're using, and what sort of metering you can do. My suggestion would be to consider the "tools" you have available, think about how they operate, and then devise an approach that will produce what you want. If you have a hand-held meter, for example, you'll likely have an easier time than if you try to use the in-camera meter. A hand-held meter that has flash capability would be even better. To illustrate, let's walk through a couple of examples.
First, you might simply use available light (no flash) and your in-camera meter. The in-camera meter would likely give you an exposure that averages the light values it sees through the lens. Depending on how much light you have coming in from a side window, for example, and what is in the scene, you may end up with an exposure time that is too slow, however. You can ask your grandmother to stay "really still" during the long-ish exposure, but you may still see some subject movement as a result. Everyone, especially older people, tend to "weave" a bit when they think their sitting still - unless they are well-braced. But, using the in-camera meter reading as your basic exposure, just bracket a couple of stops in both directions, and you'll likely have a couple of good shots.
Another alternative would be to use the flash. Instead of thinking of it as fill, I'd suggest thinking of it as the "main", and the window light as fill. Again, I'd suggest bouncing it off a large piece of white foam core placed to the side. If you don't have a flash meter, measure the distance fron the flash to the foam core, and add the distance from the foam core to your subject, and then use the guide number of the flash unit to calculate your f-stop. (Divide the guide number by the distance to get the f-stop.) The f-stop will determine the exposure from the flash, and your shutter speed will control how much the ambient light contributes to the total exposure (longer exposure = more ambient light contribution). The objective is to get the balance you want between the two. Obviously, it will be best if you can put the flash on a separate stand, and your camera on a tripod. But, if you have a long enough sync cord, you can hand-hold the flash if need be, and operate the on-tripod camera with a shutter cable. Again, I'd bracket widely to get a variety of exposure combinations from which to choose the ideal negative.
A third alternative (assuming your flash unit has a swivle head, an automatic exposure sensor, and exposure compensation features) would be to use the in-flash compensation adjustment to control the flash contribution. Here, you'd set the exposure based on your in-camera metering of the ambient light, set the flash on auto with the sensor pointing at the subject, aim the flash head at the reflector panel, and adjust the in-flash compensation to some "minus" value, bracketing with values like -1, -1.5, -2, etc. to get different ambient/flash ratios.
