NEWB TT length question

I am much more familiar with offroad geometry than road/gravel geometry. I am building myself a gravel road bike with offroad drop bars. I have a great fit on my MTB, so how much shorter would the recommendation be for the TT length compared to the 3/4" riser setup I currently use to get a similar riding position? I don't need perfect, just a starting point,as this is a practice frame, and I can adjust as needed as I get closer to my best fit through my next few practice frames. I read 6cm in another thread, is this about right?

Willie1- Could you see how an existing drop bar bike fits you and modifie from that? Or you could measure the placement of your wrist on ATB bars and drop bars, then work back to your stem. Push the math to come up with a TT length that mimics the wrist to hips. Just a couple of ideas. Andy.