Do you mean mixture?
When changing cruise power setting, always enrich reasonably, set new power setting, then lean back until your RPM starts dropping, push back in a little until you regain RPM and push a little more. This all for a given prop RPM setting.
Usually you don' t lean by percentages but some aircraft' s POH' s or operators manuals recommend leaning by a certain distance in inches.
For the correct technique relevant to your aircraft, check your POH.