TXT SPF Record

[YesM]

Hello,

I have searched this forum for answers on SPF and although lot's of questions, it seems no real answers.

I switched server and previously the SPF record was:
v=spf1 a mx include:mailxxxhost.com.cn ?all

We still use the existing host for mail services, so under Domains > DNS Settings > I added:

domain.com. TXT v=spf1 a mx include:mailxxhost.com.cn ?all

1/ Is that largely ok?

2/ I have read mixed ideas about the ?all - should this be something more restrictive?

3/ One of our ISP providers insist on sending through the SMTP, smtp.i-cable.com
How would I add that to the SPF record?

Thanks,
Andy