Hi Kim, you have most of the answers here, I will try to add a bit... The problem is you are trying to combine a couple of techniques, lets work through them.
Out the box the camera has a nice zoom from 28- 140mm. The normal optimum close focus is at the wide end, but at the cost of working distance, so basically you are right on top of the subject.
Using close up lenses allows you to go to the long end of the zoom and focus closer, depending on the strength of the dioptre, the greater the value, the greater the magnification, but at the cost of your working distance. Therefore for traditional close up work, most people tend to go for filters in a range of +2, +48 and +8 as these complement the zoom range of the basic camera.
A +0.75 dioptre is far more useful when you have a much higher level of zoom. So for example some other compacts have lenses which zoom out beyond 300mm. The problem is that at these focal lengths, the cameras and lenses do not focus very close. Therefore by adding this sort of filter, it allows the lens to focus a lot closer, so you gain a good level of magnification from the extra focal length, but also maintain a good working distance. The other benefit of this technique is that it is a way of reducing the impact of the excessive depth of field that compact camera produce, long lenses have much narrower depth of fields and may be useful when you want to isolate a subject from its background, great for portraits when you can remove or reduce the impact of distracting backgrounds.
OK so now bring in the teleconverter, the Raynox DCR 154 is a 1.54x converter, which takes the normal zoom to 43mm to 216mm, which is not extreme, however, going into the digital zoom (or crop!!!) will take this up to 862mm equivalent. While I would not recommend working at 20x (it works very well, but needs good technique to use successfully) however, a small bit of digital zoom will go a long way, the camera normally works with a 5x zoom range, extending this to x7 will take the max focal length to 302mm, fitting the +0.75 will now allow this lens to focus at (same old formula 100cm / 0.75 =) 133cm or 52 inches which is very useful. The problem is that the depth of field gets very shallow, so the tripod and focus rail is a great way to go as this will allow you to get better results and far more keepers, the down side is that you lose the flexibility of being able to move quickly and follow a subject around a bush, but you can get lots of simple flash on the subject.
As a rule of thumb I try to limit stacking my filter to 2 x units, if you want max magnification use the DCR - 250 and the Elpro3, fit the strongest unit first, plus a nudge of digital zoom if needed.
You may find that some of these bits are not totally compatible and you will need to experiment, you may find that some combinations will produce far better results than others, there may not be any individual problems with bits of kit, but when combined you can get interesting effects. It get far more complex when you are using wide-angle adaptors, but that is another story.
Hopefully this helps, the bottom line is that it is not all about getting the max magnification, but it is about understanding what the different bits will do.
