Put common code in a single Script and Include it from others ...
Do not tick the 'Allow Extenal Access' checkbox unless it is necessary for the Script.
The set of services permitted using 'External Access' are:
Configure any firewalls protecting these services to accept connections from the Workbooks Service's IP address ranges:
In addition (but not required), the Workbooks service delivery team use separate infrastructure and occasionally access from their IP address ranges may be useful. These are 188.8.131.52/29 (i.e. the addresses 184.108.40.206 through 220.127.116.11) and 18.104.22.168/28 (i.e. the addresses 22.214.171.124 through 126.96.36.199).