Calculating the balance of xpub I have a bitcoin cash xpub 


I can derive Internal and external addresses 
I have a full node so I
can check each individual addresses


My Question

01 - how do I calculate the balance ?

02 - do I need to combine the external and internal unspent amount ?

Please advice
Regular nodes, by default, only maintain a balance tally for addresses in its own wallet. Your options are to either add every address to the wallet as a watch-only address, or to build your own index on top of the node by scanning each block and extracting the data from it.

There are patched versions of nodes available, such as the ones backing insight explorers, that also maintain an address index to speed explorer operations up. However, these versions are rarely up to date.