Calculating bit score: How do you find lambda and K?

To calculate bitscore from score you can use this equation: $$S' = (lambda*S - ln(K)) / ln(2)$$

If I am trying to manually calculate the bitscore of an HSP of a pairwise blastn alignment, and I know the alignment score, how do I calculate the parameters K and lambda?

Is there a more simple equation for pairwise nucleotide alignments?

You can find these tables (from line 589 for blastn) in the file c++/src/algo/blast/core/blast_stat.c from the blast source package.