You need to calculate the melting temperature only for the part of primer that anneals to the plasmid, depending on GC content that would take up somewhere between half the primer, to one third, the rest of the sequence is your overhang.
I think the length of primer that can be synthesized is the limiting factor here.
I tried homologous cloning using inserts which had overhang that is complementary to the vector, so far i've tried up to 40bp overhang with no problem.