reference

code  unresolved bases 
M     AC 
R     AG 
W     AT 
S     CG 
Y     CT 
K     GT 
V     ACG 
H     ACT 
D     AGT 
B     CGT 
N     ACGT

The thumbrule for calculating the annealing temperature for a PCR primer is

Tm (°C) = 81.5 + 0.41(%GC) - (675/N)

of things I always forget...