e.g. to compute the expected check digit 7 for: 70651122?
digit  7  0  6  5  1  1  2  2  ?  

multiplier  1  2  1  2  1  2  1  2  
result  7  0  6  10 1 
1  2  2  4  horizonal sum = 23 
Note that the two digits of the multiplication results must be added before doing the sum.
Here is method more amenable to computer calculation:
e.g. to compute see if the check digit is as expected: 706511227
digit  7  0  6  5  1  1  2  2  7  

multiplier  1  z  1  z  1  z  1  z  1  
result  7  0  6  1  1  2  2  4  7  horizonal sum = 30 
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