No real answer to your question, I see. I'm in the same situation and would like to bring by stern up by adding weight to the bow. Haven't done it yet. I don't think I want all chain. Maybe just another 50 feet.
You could calculate or at least estimate the amount of chain you could carry (if no one answers your actual question) by removing the chain you have and putting it into a bag or box that you can measure the volume of. Then measure the volume of your anchor locker and see how many times larger it is. By volume, I mean the actual space, perhaps in cubic inches, that it takes up. For example, if the locker is 3 times larger than the space taken up by your 50 feet of chain, it should hold 150 feet. But you probably don't want to FILL that space; on my boat, if I put out all of my chain and then another 25 feet of rode, it invariably gets caught up in a pile under the windlass and I have to get down in there and move it aside to get it all back in the locker. (Not a big deal when you're used to it, but unpleasant in the rain.)
I've tried lightening up the back of the boat by shifting items forward or removing them from the boat. (I recently sacrificed a Honda generator I was carrying for emergencies but never used in 18 months.) But let's be real: we only have so much space forward and there are items we need to be able to get to without lifting the bed in the forward cabin. And I occasionally have to put a person in the Cave so I don't use it to store much. I don't like the fact that my boat -- and all the Ranger Tugs I've seen at Rendezvous and elsewhere -- ride low in the back. It's a problem to be resolved and I think adding more weight up front is a possible solution.
Good luck!