Basically you will soon know if you've got too much line in the air for your, or more likely, the rod's capabilities. The rod will just start to feel soggy in the hand during the cast and the line speed will drop as will the line in the air.
With weight forward lines the general idea is a metre into the running line. With DT lines you don't have that convenient transition between head and running line so you have to wait for that rubbery feeling that tells you you'd better stop now and start again.
I used to make up my own shooting heads many years ago. I'd buy DT mill ends, cut them in half and then cast with the half line not attached to any running line. Just a question of false cast until that soggy feeling started then pull the line back a couple of feet and cast again until it all felt comfortable. Then lay the rod down on the ground and cut the line a foot outside of the tip ring, attach the running line and give it a go. If it felt right then do the nail knot and go fishing. If not then fidget around until it did.