How to estimate amount of time for a project

BC (best case) + WC (worse case) + 4ML (most likely) / 6
or
(BC + WC + 4ML) /6

Example: Number of months needed to build a new web site:

BC=4 + WC=16 + 4(ML=8) /6
4 + 16 + 4(8) /6
52 / 6
8.6 months