CREATE TABLE table_31303 ("Season" TEXT, "Ave. Attendance" FLOAT, "+/- ave. on prev. season" TEXT, "Competition" TEXT, "Occupier" TEXT)

What is the least value for average attendance when average on previous season is +644?
SELECT MIN("Ave. Attendance") FROM table_31303 WHERE "+/- ave. on prev. season" = '+644'
SELECT MIN("Ave. Attendance") FROM "table_31303" WHERE "+/- ave. on prev. season" = '+644'