There are two lens reversing strategies for macro work.
1) Reverse mount the lens on the camera. This usually moves the lens further from the sensor resulting in a larger image. The lens' focal length must be shorter than 60mm or so for this to work well, if at all; the adapter is called a
(Pentax K) Reversing Ring MACRO REVERSING RING 49mm FOR PENTAX K NEW IN BOX - eBay (item 160356122639 end time Dec-08-10 18:28:10 PST)
Reverse mounting a fixed focal length lens results in a macro lens that focuses over a small distance with essentially one magnification and you don't know what it will be 'til you try it.
It is practical to reverse mount the 18-55 zoom kit lens and get a range of magnifications. Here are example shots of a laptop screen (0.25mm repeat pattern):
Notice the piece of plastic tubing to control the aperture lever. Also notice the small working distance (clearance).
Reverse mounting your long zoom onto the camera most likely won't work. The lens will be too close to the camera.
2) You can also reverse mount one lens on another. This has the effect of changing the focal length of the combined lenses; the magnification resulting is original_focal_length/stacked_focal_length; for example a 50mm lens stacked onto a 100mm lens gives a magnification of about 2. The stacking is done with a filter ring with 2 sets of male threads called
(52mm) Reversing Ring Male 52mm-52mm 52-52 mm Macro Reverse Ring / reversing - eBay (item 180420594619 end time Dec-10-10 08:40:26 PST) . You must be able to control the stacked lens' aperture by some scheme or other.
The working distance will be about the focal length of the stacked lens; in the above example, about 2".
Scheme 2) is fundamentally no different from stacking a close-up lens like a Raynox, etc on your primary lens. Often the smaller aperture of a reversed lens leads to vignetting - this is less of a problem with close-up lenses like the Raynox because they are pretty large. Following are the upper left quarter of macro photos showing vignetting for the stacked lens case:
Hope this helps,
Dave
PS The most effective thing for you to do I think is to buy a $55 Raynox DCR 150 (208mm) & put it on your 70-300mm lens. You'll preserve the features of your lens (even P-TTL automatic flash if it works with your lens now) and get magnifications from small to about 2X with a working distance of about 8".
RAYNOX DCR-150 MACRO CLOSE-UP LENS 52mm 55mm 58mm 67mm - eBay (item 360303820136 end time Nov-22-10 23:06:46 PST)