Ok, I've gone through the calendar, and I have the following:
Total customers on the database is actually 449.
The previous figure was from an old database export.
320 customers to generate £54k.
Customers per month, Jul 2011 onwards:
Jul: 23
Aug: 25
Sept: 26
Oct: 19
Nov: 22
Dec: 15
Jan: 23
Feb: 29
Mar: 33
Apr: 34
May: 35
Jun: 26
Jul: 31