When using a round endmill, it is often impossible to cut all details since the endmill diameter will make it impossible to reach into some areas.

One way to address this is to cut said details using a V-carve since the endmill has a potentially zero size.

This project exemplifies these difficulties if one chooses a 1/4" endmill:

as shown in the toolpath window:

The first consideration is how deep one can cut (in this instance the stock material is 1/4" deep) for a given width — one can either do the geometrical calculations to determine this, or draw it up:

which shows that we need to keep things less than 1/2" wide.

Mark the angles on the 2nd picture? Worth noting the angle of your vbit will determine how wide/deep you can go. 90 degree bits are obviously the easiest to calculate.