SPP and SPS configuration in different countries

We have a customer who has many Data Centers in different countries. They are probably buying 3 SPPs and 3 SPSs. They want to know what is the best placement of the devices (maybe they will all be virtual appliances) to have the best operation in terms of bandwidth, latency, performance and all that needs to be considered. Also, we need to know how to route traffic to one or the other SPP (with its associated SPS), considering the distance of the country from the Data Centre to have the best performance.

Is there any documentation on this? Is there any recommendation on the subject? What is the best way to proceed and what to consider regarding this need? Thank you for any suggestions that will be valuable to us. This is a big customer and we would like to implement the architecture in the best way and achieve the best performance.

thanks a lot