Lets get practical.
The object of the exercise is to meet a climb gradient, usually published so you don't hit an obstacle. So, if your are in a twin and one engine decides not to help, amongst other things that are going on you are not going to get out your whizz wheel or remember some formula from long past training days.
All you want to know is what is the minimum ROC to clear the gradient so you need GROUND SPEED X GRADIENT° = ROC you will need, look at VSI, make a decision.
All good. What about decent? The rule of thumb for 3degrees is GS x 10 / 2 = RODecent. Why doesn't the ROC formula work when you are coming down?