Help estimating database and WAL size

We are preparing a PostgreSQL database for production usage and we need
to estimate the storage size for this database. We're a team of
developers with low expertise on database administration, so we are
doing research, reading manuals and using our general IT knowledge to
achieve this.

We have actual data to migrate to this database and some rough
estimations of growth. For the sake of the example, let's say we have a
estimation of growth of 50% per year.

The point is: what's the general proper technique for doing a good size
estimation?

We are estimating the storage usage by the following rules. Topics where
we need advice are marked with ** asterisks **. Feedback on the whole
process is more than welcome.

1) Estimate the size of each table
1.1) Discover the actual size of each row.
- For fields with a fixed size (like bigint, char, etc) we used
the sizes described in the documentation
- For fields with a dynamic size (like text) we estimated the
string length and used the function select pg_column_size('expected text
here'::text)
- We added 4 more bytes for the OID that PostgreSQL uses internally
1.2) Multiply the size of each row by the number of estimated rows
** Do I need to consider any overhead here, like row or table
metadata? **

2) Estimate the size of each table index
** Don't know how to estimate this, need advice here **

3) Estimate the size of the transaction log
** We've got no idea how to estimate this, need advice **

4) Estimate the size of the backups (full and incremental)
** Don't know how to estimate this, need advice here **

5) Sum all the estimates for the actual minimum size

6) Apply a factor of 1.5x (the 50% growth) to the sum of the estimates
1, 2 and 4 for the minimum size after 1 year

7) Apply an overall factor of 1.2 ~ 1.4 (20% to 40% more) to estimates 5
and 6 for a good safety margin

I know the rules got pretty extensive, please let me know if you need
more data or examples for a better understanding.

We've also posted this question to
http://dba.stackexchange.com/q/25617/10166

Thanks in advance,
Daniel Serodio