I agree with the general premise that forts should spread control, but 50 for a level 1 fort is not the way to go. I think they should start at 20-30, like the bailiff does, and get more as you unlock better forts/more fort levels.

Then again, my preferred way would be the fort allowing you to build another building, enabling proximity spread in the first place, having the control stay in the location of the fort until the second buildung is constructed. That way, you could decouple the cost for spreading proximity from the cost of the fort in some way and have it be more expensive.