Charges which can be expressed as an ongoing percentage of the investment value are simply added together. Charges which cannot be expressed as an ongoing percentage are amortised over the relevant period and disclosed accordingly.
The calculation takes account of the type and timing of the investment (lump sum or debit order) and whether there are any initial or ongoing fees applicable, as well as costs incurred in the administration and management of the product.
This may entail forecasting the growth of your investment at a predefined rate, but note this does not mean your investment will necessarily grow at this rate. The detailed calculation methodology is available on request.