Fill formula according Column c

hello
i using below code to fill formula down end........

but i want formula fill only according to Column C
i mean if column c4:c15 have data then fill only till C15

thanx

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Thanx MOD alansidman
and
thanx JOHN H. DAVIS

both method is working perfect

thanx alot both of u
i will add reputation both of u ?

but MOD alansidman ur method is working so fast