I asked about this a few months ago, and what I was told is that generally you have enough material in most cases. With the skin being elastic, it's easy to stretch it and get good depth as well. In your cast, that erect length, I'm not totally sure if doctors use that or if they use just a stretched flaccid measurement. When it's not erect, you might notice you can pull some of the skin further than erect which is sort of what the surgeon can do. And lots of transwomen, especially who started young and have surgery, usually have no complication and have good depth and just in general good results. Having not been circumcised also it a benefit for SRS too! I'm in the same boat as you with the stuff down stairs (though losing a bit of weight might help in my case), but honestly you should be, but if you aren't, then there is always different methods that can be used to achieve good depth.