Formula:

Ta(1) = 4(G+C) + 2(A+T) - 5 <= 20 bp

Ta(2) = (62.3 + 0.41*(%GC)-500/length)-5 > 20 bp

Ta(3) = (81.5 + 0.41*(%GC)-675/length)-%Mismatch

Source: Akami Quick PCR Guide.pdf