I am sure others will have their own opinion, but I believe your boundaries are far too generous for Midtown. You really want to be easy walking distance to the commercial amenities on North Monroe and Thomasville. And even though the Centre of Tallahassee is being renovated, I wouldn't include it in the equation. It will still be primarily car oriented and trying to cross North Monroe/John Knox in that area is anything but a pedestrian friendly experience. (In fact, far too much of North Monroe is unfriendly to pedestrians, and, to be frank, an ugly assemblage of chain stores, pawn shops and check cashing joints).
I think Midtown brings together a combination of older building stock, mature trees and landscaping, walkability, and a higher concentration of independent businesses. And you HAVE to include Lake Ella as the jewel in the crown. So I would set the boundaries as follows:
Magnolia/Centerville on the east;
Betton/Bradford/North Monroe/Monticello on the north (though I would also include Glendale and Dellview Drive);
Monticello/Gibbs south to 4th Ave on the west, then MLK south to Tennessee on the west;
Tennessee on the south.