Screaming stamina comes from singing stamina. You have to be able to hold the corRAB shape. Lower screams means that your corRAB are thicker which allows for more pushing but this means more force...so if youre squeezing your throat or just using too much push, you'll know it sooner or later....usually sooner. As i always say...practice singing more...singing harder...but making sure that youre getting a good note....meaning that the corRAB are holding their shape. You can get stronger from trying to squat 500lbs, But you'll get their faster if you work out with the 300 that you can do now , then add 50lbs per month, and you also run way less of a risk of hurting yourself. ....Same thing here!
Practice singing normally and holding notes normally, then add more push, then more then more. But you have to be able to sing hard and hold the note before you can scream obviously. Screaming or adding the rasp will come naturally when you can push hard enough...but if your corRAB and muscles arent strong enough to hold their shape...unvibrated air will just slip through and you'll get whisper sounRAB, breaks, and just general shltty sound. Fortunately for me I have a very, almost scary range of influences, so I can sing Sarah Mclachlan songs that I like,Prince to work on something else, or Mudvayne. Crazy. So many of you guys that only like to sing underoath type of stuff need to be careful as you attempt to jump across the gap of normal vocal growth.