and doesn't require any mental math to be on profile like your 3 times altitude.
Even if you're not using VNAV for descent you can still follow the pathing information if gives you, you don't need to be in VNAV for the path to be accurate.
I would be careful how you interpret that advice. Always cross check your vertical profile with basic maths. The VNAV profile can be incorrect through finger trouble 20 minutes ago,or through a computer fault.
Also, when it is not appropriate to be using vnav ( 4 miles from an 8 nm final on a base leg, on vectors and the controller cuts you in) then you will be used to doing the calcs and won’t be tempted to go digging in the CDU.