Is MySQL Cluster a right choice?
When evaluating clustered solutions you need to think about the read/write profile of your application and match that against available solutions.

For a heavy read based profile of an application, you would be able to scale up reads by having multiple MySQL replicas off of your MySQL master-master replication setup.

For write heavy profile, you are implementing some of the obvious choices in master-master replication.

When choosing between clustered MySQL options there are two technology options: NDB based Oracle MySQL Cluster and Galera based options from Percona (XtraDB Cluster) and MariaDB (MariaDB Cluster), and Galera Cluster for MySQL.  They are architected differently and have different advantages and disadvantages.

A good webinar on the distinction is here:

http://severalnines.com/blog/galera-cluster-mysql-vs-mysql-ndb-cluster-high-level-comparison-webinar-replay-slides

The formula mentioned is primarily used in measuring the projected per node requirements for a NDB cluster.  Originally MySQL Cluster, previously NDB Cluster, was a memory only database, which required all of the data to reside in the memory of available nodes.  If all your data was required to stay in memory, then if you didn't enough RAM to hold all your data, you are under provisioned in terms of hardware.

Nowadays, each node in MySQL cluster, namely a data node, is a MySQL instance.  Even though you should keep your data in memory for optimal performance, it is not required.  So, basically if you don't "obey" the formulas, you will end up dropping down to disk and would have to deal with all the IO performance of the particular hardware available to your nodes.