I am going through this process now. I have had 3 dojos in the past so I like to think I have acquired some "lessons learned" as well as from others I've observed.
First - it's great that you and the Dojo Cho are on good terms. You leaving shouldn't mean that you ARE leaving. I plan on having my former colleagues as guests and I still visit there occasionally. The last thing we need in aikido is more division.
Second - what I am doing is starting small. I found space in a local health club that doesn't have an art similar to aikido so there is not competition factor. They do have karate but they either meet on different days or there is a good amount of time between the two classes. I found when you share an actual dojo with an incumbent art, there is going to be bad blood. Usually from students, sometimes from instructors.
I don't pay rent. I have a 60/40 split. I set the prices. It's good because I don't have to collect money or keep track of attendance - that's done at the front desk. So it's like I have an admin.
Third - I'm buying mats as needed. I'm starting out enough to cover a 16x16 or 20x20 space. So as students join they basically pay for their mat space.
Fourth - Time. This was the big issue for me. I work, I'm a part time student, have a family, and must still train with my Shihan. You can check my schedule at
http://www.webzue.net/woodbridgeaikido.htm
to see how I have alloted my time. This seems like it will work for me but time will tell. I also considered just twice a week classes.
I found out from my past experience that having my own place helped my progress tremendously. I get to work closer to home and more often.