Resource planning in order to minimize slack will have to introduce buffer time. Or if I may say so, reinvent slack.
There's an interesting book by Tom DeMarco called Slack.
A main point in the book is:
if working is modeled as having an inbox queing of work to be done and an outbox of finnished work and if a company consists of many such working units then maximum efficiency defiend as all workers are always occupied is acheived when all inboxes are always full . However, having a full inbox means incoming tasks are queued and hence if all inboxes are full in a company every single task that has to pass many inboxes to be completed will inevitably spend a long time waiting in inboxes . Therefore, a modern highly effective company is a company which cannot respond quickly.
Slack in an organization is requried, DeMarco argues, in order to respond quickly to issues (and drive the necessary change of a company required of the changing world, but thats another story).
Observations on MS Project
I've been involved in a few projects utilizing resource planning tools like MS Project and similar tools [1] . Used in a traditional company being effective MS Project has the vital role of making shure the inboxes are never empty but also never too full.
Advanced project leaders try to estimate and compute the time a specific task will spend in each inbox and in each process step. But such planning can only reduce the lead time of an expected task by minimizing planned waiting time. Unexpected events, additional tasks etc. can rappidly defeat any such plan. Therefore all project managers learn to introduce buffer time. That is, one plans for extra time somewhere because the detailed plan crafted to reduce lead time while keeping inboxes full indicated (surprisingly) that unexpected events may give rise to heavilly increased lead times due to dependencies.
And buffer time is...?
[1] Other usefull tools are Mr Project and Open Project.