The descent rate is the idle thrust gliding rate. Steepness depends on the chosen speed, the schedule 250/280/M.78 mentioned above for climb works perfectly too. Practically speaking, you do not pick a decent rate, you choose the ToD point. Perhaps at around 5000' AAL 220 kt is prudent for manoeuvring the terminal procedures and no more than -1500 fpm to avoid undue TCAS alerts / inconveniently large thrust changes. When you see the A/C not being able to dive -1500, that's realistic too.