Late fees calculations over 30 days

[Mak K.]

My tenant has aug rent past due. I charge $25/day late fee.  She is also going to be few days late for september.

So for calculation purposes lets take 5th September as example. So since now its two months of late fees as of sept 5th, do i charge $25/d for August late fee plus $25/d for september? 

I mean does late fee gets doubled up ?  or does late fee remains flat no matter what?

location- TX

Lease contract- texas standard TREC lease agreement

note- there is a reason why tenant is late and I am ok in this scenario of tenant being late.