I have given it more work. Plus got some ideas from Ron's approach

this assumes a cell has the SIN in in the format ###-###-###
This will give the check digit
=SUM(INT((MID(A1,{2,5,7,10},1))*2/10))+SUM(MOD((MID(A1,{1,2,3,5,6,7,9,10,11},1))*{1, 2,1,2,1,2,1,2,1},10))

I also like Ron's
=mod(A2,10)=0 to give a True False return for the flag.

hth RES