There is no official guidance on "how far back should we go" on any SAR subject. Go back as far as you need to to support your suspicions, but there is no point in attempting to locate the source of the Mississippi.
Search on the term "kiting;" there have been prior discussions about how to calculate the amount. Personally:
1) I agree with rlcarey, your concern is about checks deposited, not checks written, but
2) the right or wrong of the calcuation makes little or no practical difference.
The aggregate dollar amount of a kite is more theoretical than practical if your bank has yet to lose any money.
What Happy said:
1) close account
2) worry about SAR.
Closing the account is what's important. Filing the SAR is nothing but the details.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.