I don't think your stem could be 150mm. That would be extremely long! You might want to measure it (center to center). It's probably somewhere between 70mm and 110mm. Sometimes bike shops will let you borrow various sizes of stems so that you can see what's comfortable for you. I had a bike with a 90mm stem, but found that the reach was a bit too long. I changed to a 70mm stem, but I didn't really like how it changed the handling. I ended up getting a frame with a shorter top tube that I could run a 90mm stem on. It really is just trial and error. You'll probably have to try a couple of stem sizes and change the stack height of your spacers until you totally dial in your fit. Good luck!