you can do it a couple of ways, but the easiest to me is to look into the master as you put that master back on. you can see the piston move in the hole inside. I adjust it to see it move then back it off till it doesn't move when I put the mc back on flush.

BUT- lets back up.
Did you only change that steel line? nothing else? Why did you change it?

IF that is the only thing you did, you are just messing your self up checking these other things. IF the brakes worked fine before and you only changed that line you need to be checking things down stream to see if something couldn't handle the change before you start messing up stream again.