I recently removed the crumbling concrete curbing from around my front porch flower bed. This weekend I plan to create the form and pour a new curb for the bed. My question is: When I build the mold, should the mold overlap the existing concrete edge some, allowing the existing concrete edge to act as a kind of lip that part of the curb will sit on, or should I try to bend the mold to perfectly create a seam along the existing concrete's edge? I feel as though overlapping would ensure a cleaner finish, but it puts me at risk of cracks. I feel as though trying to match the seam exactly will leave a messier finish but be more crack resistant. I do have a diamond bladed angle grinder at hand to clean up existing concrete edges. As for climate- This is in Philadelphia.