You could do it a couple of ways, depending on which actually best models
the actual physical process. If you mean Task B can start 15 days after
Task A starts, link A to B Start-to-Start and add a 15 day lag time. If you
mean Task B will be able to start 5 days before A ends, link A to B FS and
add a 5 day lead time. If you mean B should start when A is 3/4 done, link
A to B FS with a 25% lead time - in that case, if the duration of A changes,
the exact amount of overlap will adjust so that it will always be 25% of A's
duration, whatever it turns out to be.
Remember that links model physical requirments, not preferred time
relationships. It's not that you want B to start 15 days after A, it's that
A is producing something required by B and that something won't exist until
A has been running for a certain period of time or has reached a certain
level of completion. Also, the link doesn't specify when B WILL start, it
specifys the earliest time B will BE ABLE to start .... there's a
difference. Any number of other factors (like the only person who has the
skills to do B is on vacation the day when A gets to the point B can start,
for example) might make it actually start later than that no matter what the
link says.
HTH
