The phrase "a solid rule" is correct and usable in written English.
It can be used to refer to a principle or guideline that is reliable and effective.
Example: "In our organization, we have a solid rule that all team members must communicate openly and honestly."
Alternatives: "a strong guideline" or "a firm principle".
(He refused all selfies – a solid rule about dignity. Non-selfie photos were fine).
There's a solid rule in legal lore: "Justice delayed is justice denied".
The Roadhouse, Newton Street, Thu Marc Rowlands BournemashupbaseDJ Cursa DJ Cursa has a solid rule regarding these blends of classic tunes with crunching beats: if you don't make them yourself, don't play them out.
His ineffective complaints about the White House's scorched-earth strategy illustrate a solid rule of American politics: Never, ever, demand that your opponent apologize for a campaign attack unless you have some reasonable expectation that he actually might.
A solid rule for Sunday night at Reading is to try your hardest to stay awake, because let me tell you now, the potential for antics is off the scale.
Being classy is not exactly a solid rule to becoming preppy.
It seems to me that there's a fairly solid rule, that applies to almost any question: what you want to believe is almost always wrong.
A FEW solid rules would have been helpful, but without knowing how either relationship might progress, we didn't know how to set them.
The queen presented gifts to President and Mrs. Bush, a solid silver rule engraved with her royal cipher and the president's seal, which listed, on the reverse side, all the previous presidents.
The doctrine was widely seen as unfair, for various reasons - it made no exception for cases where the parties to a contract obviously intended for it to be enforced by a third party, and it was so inconsistently applied that it provided no solid rule and was therefore "bad" law.
