While not a rule, this profile is standard practice at my company.
Break the descent into 3 phases:
Plan a 3° descent angle for phase 1.
Phase 3, is the approach, so you will have some sort of 3° guidance.
Phase 2, we fly whatever is needed to connect phase 1 to phase 3
There are multiple ways to create a 30nm fix. One method is to go the FIX page and put a 30NM ring around the airport. Pressing the LSK for the fix will put a fix in the scratch pad that is the airport minus 30nm along the route of flight. Put this fix in the LEGS page. Set the descent angle of that fix as 3.0° and put an altitude of 10,000' above the field elevation.
The FMS will create a top of descent (TOD) point that is a 3° glidepath to the 30nm fix. Approaching 50nm to the TOD, The time and distance to the point will populate in green on the top right of the MFD. At some point, ATC will give pilots discretion to a lower altitude. Once a lower altitude is set the altitude preselect, the descent rate and angle will populate in green on the MFD as well. Within 1000' of the 3° glidepath (3nm from the TOD), the advisory VNAV should populate on the right side of the PFD. At the TOD point, select VSPD and select the advised vertical speed. Adjust thrust to follow the speed profile described above.
At 12,000', set thrust to idle, set vertical speed as 1200fpm, and select a speed of 250kts. Monitor not to exceed 250kts below 10,000'.
For perspective, this is what 3° really looks like to scale.
Now we aim to set ourselves up on a 3° glideslope to the runway. Let's take for example KMHK, RNAV runway 21. The FAF is AGOKE at 2800'. The IAF is BIRME at 3100'. The FAA chart's profile view would lead you to believe there is a constant angle of descent from BIRME through AGOKE to the runway but this is misleading. There is only a 300' difference despite the two fixes being 6NM apart. Using our 300' per NM rule, we wish to be at BIRME at 300' x 6NM = 1800' above AGOKE. During our approach briefing, we should have penciled in 4600' for BIRME. At my company, we are not allowed to modify an approach in the FMS, but if this were a visual backed up by the RNAV, then I would have set the altitude in the FMS as well.
From the 30NM/10,000' fix, it is a perfect 3° slope to the runway. At 250kts, expect a descent rate of ~1300fpm.
From the 30NM/10,000' fix to the base fix at 3000' AFE (~10NM at 3°), is a 2° glideslope. At 250kts, expect ~900fpm descent
When the IF of the approach is on the other side of the airport from our direction, our 30NM/10,000' fix is about 40NM away and from the IF fix at 3000' AFE. The descent angle is 1.5°, expect a descent rate of ~700fpm at 250kts.
The easiest way of course is to ask ATC for direct to the IF, now we can use the DIR INT page on the FMS to descend along a 3° glideslope to the FAF. (The IF vertical guidance may not be correct if it still has the published altitude which is lower than the 3°).
An alternative method is to use the range to altitude select. As we descend, ATC will continue to clear us to lower altitudes, but these altitudes will usually be in even 1000' increments. So the range to altitude select on the MFD won't lineup with our altitude of 4600' at BIRME. The actual clearance might be more like 4000', so we will have to eyeball the range to altitude select slightly past BIRME.
Plan to cross the IF at 200kts. If the speed trend indicator points down 5kts per 10seconds, it takes 6NM to slow from 250 to 200. I usually start slowing 10NM prior. In order to slow 5kts/10sec, the descent rate must be reduced to ~1000fpm at idle. The aircraft will trend above a 3° slope, however, once speed is reduced and flaps come out, the descent rate can be increased and 3° glidepath recaptured.
For a 3° glideslope at 200kts, expect a descent rate of ~1000fpm.
When approaching the IF at a right angle and 200kts, the FMS seems to anticipate the turn at about 1.5NM.