Because of the way slides work, I think no matter what you'll have very dark images. ISO 1000 is roughly 3 1/4 stops push from 100. Rounding off the 1/4 is fine - in this case it won't make a difference. I think one reason why there's not much information about pushing slide film three or more stops is that it is generally unadvisable. One reason why it is considered unadvisable is that with that big of a push, you will almost certainly get color shifts, and you'll also have detail-less shadow areas (and a lot more of them) no matter what you do at that point, because of the way transparency films work. Transparency films work by dye destruction - they get thinner and brighter the more you expose them. If you over-expose a slide, there is no possible way to salvage it because the information is just gone. Conversely, if you under-expose a little, you increase the saturation of colors and deepen your shadows. If you grossly underexpose, you have solid blacks that have no detail. I wish you the best of luck with it, and I hope you can salvage some images.