4-5-6 7-8-9 rule

Easily remember the approximate Oxygen-Hemoglobin dissociation curve as follows:

PaO2 40 corresponds to SaO2 70%

PaO2 50 corresponds to SaO2 80%

PaO2 60 corresponds to SaO2 90%

This rule along with a lot of other handy RT knowledge can be found here

Read more about PaO2 vs SaO2 at airwayjedi.com here