Draw half of your dome profile then do a revolve surface revolve.
1. Change your view to the Front View (X/Z plane) and then change your USC plane to Front (X/Z). This gets your dome to rise on the Z axis.
2. Draw a circle with the center at X0, Y-2.5625 (radius - .5"). This will set the top of your dome .5" high. (Remember that you still are drawing with the values of the Top UCS, X right to left, Y up and down as you lok at the screen).
3. Draw a vertical line down the Y axis.
4. Draw a horizontal line down the X axis.
5. Trim the lines and circle to get your profile.
6. Use the surface revolve to revolve your profile into a dome.
Hopefully this helps. Step 2 through 6 are attached so that you can see the settings for the operations.