There are two lead cases on damages in patent cases.
1. General Tire v Firestone  1 WLR 819, and 2. Gerber v Lectra  RPC 383 both at first instance and at 1997 RPC 443 in the Court of Appeal.
The following general principles were set down in General Tire by Wilberforce. Any starting point must be in any tort related case to focus on the principles in Tire in relation to ‘economic’ torts in that the measure of damages is to be, so far as possible, that sum of money which will put the injured party in the same position as he would have been in if he had not sustained the wrong. There are generally three identified categories: a manufacturer losing sales, a licensor losing licensing revenue and the cases in which a notional reasonable royalty rate must be estimated.
In Gerber the court assessed the general overall approach as follows namely you start with the total number of infringing sales made by the defendant and then estimate the quantity of those infringing sales which the patentee would in fact have made. In relation to those sales, a sum for that loss is awarded- so this is easy peasy. However it may get more complicated if there are some sales which the patentee would not have made in which case a notional reasonable royalty would apply.