What do you even mean? The post you replied to said 'The CBI estimate that the net benefit to the UK of EU membership is around 4-5% of GDP or £62bn-£78bn a year. ' Those numbers at the end '£62bn-£78bn a year' are what the CBI estimate the UK benefits from a year. We get it mostly from having access to a $16.6 trillion a year Single Market of 500m people.
Our net contribution 2017 was £8.9bn. Taking the lower CBI estimate it is £62bn - £9bn = £53bn profit. Though if we're out of the EU we obviously won't be getting the CBI's estimated benefits.
If we go to WTO deals commensurate with what we have now GDP is expected to decrease by 8.8%. So that would be GDP - EU contributions - 8.8%. So 1.56tn - 8.9bn - 8.8%.
If you're as smart as you make out then you must have already known this though.