Just check if distance to go is good for your height.
The rate varies a lot when you make idle descents. But, for 3 deg slope, If you want to calculate if you are making 3 deg or not, try this, using GS:
300 kt aprox 1700 fpm
250 kt aprox 1500 fpm
200 kt aprox 1000 fpm
I use aprox GS in nm per min times 3, add two zeros and plus a 10% of that (it's like multiplying by 330)
Example.
At 300 kt you fly at 5 nm per min, so in a min, at 3deg (330 fpnm) you should descend 5x330=1650 ft.
I use it all throughout my descent so I know if I am progressing as planned or not. I have noticed that the average gradient keeps fairly constant, so I can extrapolate and predict the trend, so I know early if I will need higher speed or speedbrakes or maybe reduce rate for a while if Im getting low.
I know I make too much maths, but I like it. i also monitor still air gradient, with TAS.
which helps me check the effect of weight and speed in the gradient