I am trying to create a spreadsheet for work to validate SIN numbers.
This is the formula in it's non spreadsheet form:

http://www.pwgsc.gc.ca/compensation/ppim/ppim-5-1-e.html

SIN VALIDATION
 PURPOSE--To check the validity of a Social Insurance No. (SIN).
 REQUIREMENT--A newly acquired, or previously un-recorded SIN, should
be checked for validity prior to input so as to ensure that it will
pass the personnel-pay system computer edits.
PROCEDURES
 TO VALIDATE A SIN, PROCEED AS FOLLOWS:
   Originator
 --WRITE the SIN on a sheet of paper, e.g. 440-968-592;
 -- INSERT a check mark over the 2nd, 4th, 6th and 8th digits, as
indicated above;
 -- WRITE the SIN again, but this time doubling the digits that were
check-marked, i.e. 480-18616-5182.
 WHERE THE DOUBLING OF A SINGLE DIGIT RESULTS IN A TWO-DIGIT NUMBER,
THEN:

 IF THE SIN IS VALID THE RESULTING TOTAL MUST BE A MULTIPLE OF TEN.
 Therefore the above SIN is valid in that the total is 50.