Descent to a fixed point

[Groundbased]

Hi,

I have a question about descending to a fixed point.

When you get a clearance to be at a certain level, by say 25 miles (is that NM by the way?),
before a specific waypoint or navaid, what is the best way to meet the restriction?

Would you create a new waypoint in the FMC route and let VNAV do the work, or do
some speed/ROD calculations and then use v/s or lvl ch modes?

Also is there any tolerance in this type of clearance?, +/-

Thanks in advance for any answers.

GB