Math is Math. 1 degree is always 100 feet per 1NM.
what size plate does your book say it has. Keep in mind that even the "modern" plate will have side lobes, they just are not as pronounced as the old dish style.
if you have a 12 degree beam width, and tilt is at +1; then you are sending 5 degrees down, and 7 degrees up. That would be 500 feet per nautical mile down, and 700 feet per nautical mile up, for a total beam width of 1200 feet per nautical mile.
Getting ground clutter (ground returns) is from having the tilt too low at lower altitudes. In your post, you do not say from what altitude you are getting ground returns from, since you can get them from any altitude.
If you have a 12 inch antenna, the mathimatical formula translates into about an 8 degree total beam width, with side lobes extending up to 12 degrees total. The side lobes are not as pronounced as on older dish antenna and shouldn't cause much problem unless down low. Aircraft structure, and installation factors can make the side lobes more pronounced on different aircraft. For instance, the EMB-145/140/135 has a 12 inch plate which should be a 8 degree beam. Using the 1/1/1 formula, I have yet to see one that didn't put out 12 degrees... in spite of the company manuals saying it was 8 degrees.
Another thing to do, is to visit the website of the radar manufacturer and download the user manual.... or check the cockpit library during a longer flight. Determine what the base DB level is for the MFD display. Then find out what each tick mark on your gain control translates into in terms of DB's.
With some skillful use of the gain, you should be able to actually determine storm levels accurately. You can use the National Weather Service scales as a reference. For example. The Honeywell Primus in the EMB has a base DB of 20 decibells. For ANYTHING to show up at all, it has to return at least 20 DB's. Each tick mark on the gain control is 3 DB's. Adjust the gain control from full, down until your target just disapears. Count the number of tick marks on the gain control, multiply times 3 and add the baseline of 20 DB's. That will give you the actual DB return of the cell/area in question. Use the number of DB's as compared to the NWS scale and you now know what level storm you are looking at. Re-adjust the gain knob to display the appropriate colors.
The latest and greatest radar, not in airline service yet that I am aware of, also includes a GPS derived terrain database... the radar compares returns, does the math for you, and removes anything that is a ground return. It also does vertical sweeping as well as horizontal. The display gives TWO pictures. The traditional plan view we are all familiar with, and then underneath, a PROFILE view showing tops and bases. Really cool stuff.
Hope this helps.