Although Regula is indeed the feminine version of Regulus, the early Christian context in which the name first appeared makes the meaning "rule", "ruler" (in the sense of measuring device) much more plausible. Most bearers of the name are either Swiss or of Swiss decent, to be precise, from the Zuerich area, from Chur in the canton of Graubuenden or, more rarely, from Andermatt in the canton of Uri where the relics of the Zurich town saint were brought during the Reformation.
A beautiful name (yes, I love my name) – but if you want to give it to your child, be aware that a lot of people – especially in English – have trouble pronouncing it (or even remembering it).