This response is based on the information provided.  We suggest you discuss with your contracting team, program manager and/or legal department as appropriate. 

What is necessary to determine is what was your original acquisition strategy.  What was the documented intent when the contract was awarded?  Particularly, what dollar threshold was established for your T&M CLIN?  If you are adding additional dollars within the established or amended threshold you have no issues.  By definition, T&M amounts are estimates and we reasonably expect that we may need to add funds to an originally negotiated amount.  You do not specifically need a "75% complete letter to add funds; it is a very good basis for adding funds, but not the only way to justify adding funds.