I recommend the Cateye Astrale 8 as an inexpensive computer that has all the functions you need (including cadence). We faced the same situation with the gap between fork leg and spoke. I think, however, that shimming the sensor out to meet the spoke magnet is a better solution than stacking an inch worth of magnets together to meet the sensor. BTW, I didn't know what an Electra was so I did some Googling. Is your bike only 3 speeds? I have to tell you, there have been some debates on here about entry level tandems and what they are and what they are not but IMO you will not be happy with that bike for very long. Don't put any more miles on it until you do some research. The reason you are ok with it now is because of your relative inexperience. I haven't been able to find an Electra for less than $1200.00. For much less money than that you could get a Fuji Absolute, A KHS Tademania or a Raleigh Coupe or Companion. These are FAR better bikes than the Electra and far less expensive. FWIW.