When you want to calculate something per thousand, you are looking at how often something occurs every thousand times. For example, if you produce widgets and know your total cost is £325,000 for 400,000 widgets, you can calculate how much your cost is per thousand widgets. You can look at anything per thousand units. In order to solve the problem, you need to set two ratios equal to each other, then cross multiply. Finally, you need to solve for x.
Write out your original ratio. For example, £325,000 / 400,000 units.
Set the original ratio to equal x / 1,000. In the example, £325,000 / 400,000 = x / 1,000.
Cross-multiply the ratios. In the example, 400,000 * x equals 400,000X and £325,000 times 1,000 equals 500,000,000.
Solve for X. In the example, 500,000,000 divided by 400,000 equals £812 per thousand units.