There is just a yes/no toggle, with "no", or "yes (with ip of server)"
Neither are correct in all cases. In my case, I've offloaded SPF to a central mail server, and use include: directives.
There needs to be a third option: SPF: No. SPF: Yes (With IP of server). SPF: Yes (Custom) [edit field where user can type their own SPF=v1 etc etc string]
Either way I solved it (and many other issues) by simply defining the entire default DNS zone file instead of letting Virtualmin add entries on system creation.