From what I can gather from the info provided in the A70 manual, the A70 has a guide number (GN) of roughly 39. We can use this to determine what aperture you will need to shoot a subject at a specific distance. You'll have to work out some calculations but in Manual mode, this is the only way to do it. And it may not be exact but it's a starting point.
1. Set the camera to M mode.
2. Determine your flash to subject distance in feet. Divide 39 (the GN) by this distance to arrive at the aperture you'll need. Set the camera aperture to the one closest to the result.
3. Set the shutter speed to 1/60. It can be higher if you want to freeze people motion.
4. Take a picture and see how it looks. Select the next wider aperture if the image is too dark. Select the next smaller aperture if it is too bright. Keep doing this until the image looks good.
5. If you run out of apertures on the wide end, the subject is too far so you'll need to bring it in close or increase the ISO. If you run out of aperture on the smallest end, you will need to decrease the flash power output. See page 112 in your A70 manual on how to do this.
Hope this helps. At least you should have fun experimenting (I hope). 