[Apparently, line breaks are removed from comments, so let's try using a reply.]
Indeed, I'm using
exists:spf.%{p2}.%{d}
exists:spf.%{l}.%{p2}.%{d}
successfully for sending mail to Gmail.
This lets me enable in Internet access provider to accept mail with with a sender address in my domain by adding an A record to my DNS like
spf.provider1.com.example.com
I can be even more selective with an A record like
spf.user.provider2.com.example.com
to allow user@example.com to post his mail through his provider's mail server, so I don't need to create and manage an account for him on my mail server.