Dear Mr Pramod ,

usually thinning will be of the order of 15 % . I do not think there is any ready to use formula for estimating the thinning . Usually inspectors ask for a mock up piece for demonstrating the extent of thinning . Thickness calculation for curved plate and a flat head show huge difference naturally. You may look for some guidance on rectangular tanks in Megassey.

Until more knowledgeable answer is offered by others I hope this may be of use.

Best regards

Venugopal