Sweetheart, I am NEVER right when I try to guess at my wordcount - my longest HD fic ended up at around 40k, and I thought it was only going to be around 8k, that's how far off I was! Because the thing is, you can plan and plan all you want, but what you end up writing will always have a life of its own. So my advice would be to let it: write the scenes you have planned; some will barely make 500w, but others could end up being 5,000, and you won't know until you get to them. So write the scenes you want to write, and if it does, by some weird occurrence, happen to be under 50k by the end, you can up the wordcount then. But I honestly think that 30 scenes will have you blasting past that 50k before you even know it! <3