Why toolholder reach needs its own calculation
A deep pocket or a feature buried below the top surface of a part needs a tool and holder combination that can actually reach the bottom without the holder body or spindle nose colliding with the part or fixture first. Ordering a holder based on feature depth alone, without adding clearance, is a common way to discover mid-job that the holder is not quite long enough or that it rubs the part wall on the way down.
How the number is calculated
Required toolholder reach equals the feature or pocket depth plus a clearance margin, giving the minimum stickout the holder needs below its clamping point to reach the bottom of the feature with room to spare. The holder's own clamping length still needs to be added on top of this when specifying or ordering an extended-reach holder, since that portion sits above the working reach and does not cut into the part.
Practical tips
Build in more clearance margin for deep, narrow pockets than for shallow, open ones, since a longer reach flexes more and chatter risk goes up with unsupported tool length. When ordering an extended-reach holder specifically for a deep feature, always add the required reach calculated here to the holder's clamping length rather than treating the two as the same number, since manufacturers spec holders by total length, not by working reach alone.