No need to LAG. Summary Functions are windowed too. It reads much cleaner.
select PK_S, MonthDate,Leavers
,SUM(Leavers) OVER(ORDER BY PK_S DESC
ROWS BETWEEN CURRENT ROW AND 11 FOLLOWING) as RTM
from Headcount
order by PK_S
set statistics time,io off;
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Against stupidity the gods themselves contend in vain. -- Friedrich Schiller
Stop, children, what's that sound? Everybody look what's going down. -- Stephen Stills