Client and Server execution

Hi, I have some troubles implementing function which should work on client side and sever side

If I use &AtSever then from item form it uses client/server call, if implement it as &AtClient I could not use it at server side which I also need to. It does just simple mathematical calculations, but it is big and I do not want duplicate code for client and for server.
I have used two versions for client and for server with the same name and it work until 8.2.15. But really it seems to me that it is not a good idea to do so. How could I run the exact same part of code from server and client without copying that function?

If you cre ate function at common module with properties "Client (managed application)" and "Server", so then you can call this function at client or at server

You can look example at 1c subsystems library. There you can find common module StringFunctionsClientServer - functions of this module you can call at client or at server