One can deduce the annealing temperature of primers from its tm
tm= 4 (A+T) + 2 (G+C), once the Tm is calculated, Ta is 2 to 10 0c less than tm. If we deduct on an avg 6 0c from tm then it will give approximate range of annealing temperature and most of the time it will work. Please do check it by doing pcr.
Suppose Tm=65 then Ta=Tm-6= 65 - 6 = 59 so Ta will be around 57 to 59