How is e-value calculated in BLAST 2 sequences (BLAST+)?
You do not need the nr database for BLAST to calculate an E-value.
It will be calculated based on the alignment of n sequences.
The exact procedure is mathematically complex but if you want to read the original paper it is here:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC53667/pdf/pnas01031-0226.pdf

a simplified version you will find here:

http://homepages.ulb.ac.be/~dgonze/TEACHING/stat_scores.pdf