This app was mentioned in 1 comment, with an average of 1.00 upvote
It was designed in accordance with the ASHRAE (US) and AIRAH (AU) guides. As CIBSE doesn't publish their information freely I can't check to see if there are any regional peculiarities, but I can't see any reason they would differ (apart from possibly sizing criteria, which you can customise yourself anyway). There is now a freemium version which you can try for free, so give it a shot and see how it compares to your current calculations.