How do I set a value based upon other values? I have a content type of product, and want to set the postage cost for the item which is dependent upon length,width,height, weight but also product kind - e.g. hazardous items needing special courier etc. I have a series of prices for the various weights and size limits, so I'm wondering how can I utilise these prices for the postage field automatically.

At the moment, the prices aren't inserted in Drupal, would these prices be better as a separate content-type or as fields inside a courier content-type? If the latter would they be as easy to reference?

This is not for a commerce site, merely somewhere to manage all my stock from all the various suppliers - a product information manager.
If you have a small number of settings to store, you might consider using system_settings_form() to store them in Drupal's variables table. Take a look at the core modules that implement this function for more examples.

If the rates group logically, and/or need to be edited by users, you might make a postage_rates content type with fields on that for the information that changes in addition to any constants set by an admin using the first method. You can then add a entityreference field from you product to the postage_rate.