You should start with loading A with the first value and add the next 6 before you divide by 7.
When you do what you do in your example you start with an unknown A register adding the first value. You can of course clear the A register or load it with 0 before you start the adding(with 7 values.
Remember to check that the 7 values does not add up to more than 255(carry) for a single 8 bit value. If more, you need to use a 16bit (D=A+B) value in your computation. To use the IDIV instruction you need to use the D-register any way. IDIV-> D= (D/IX).
If you don't know if the sum of the 7 values is >255 you should use D as the accumulator. Remember to addc A, #0 \after each addition to add in the carry. Also remember to clear the carry before you add in each of the digits or use the add a,.... that does not use carry.
Good luck.