Question about AWS Calculator

I am using http://calculator.s3.amazonaws.com/index.html to calculate the
cost of RDS vs EC2. Assuming that I am going for only a 2 node setup
(master and slave), few questions:

1. In EC2 how do I indicate that I need two nodes so that I can set up
streaming replication between them for HA. Do I mention 2 as number of DB
Instance or just add two rows.

2. I understand that in EC2 I can also pick the two nodes in different
availability zones and pay for replication data transfer between those
zones. Am I right ?

3. In RDS it seems HA is built in without the need to mention nodes, be is
same zone or MZ. So how does user pick number of read replicas.

thanks.