Currently the SPI Index does not function properly for tasks that start later than intended.
For example, if I have a Baselined task of 1 week duration (40 Planned Hours) that was supposed to start 2 weeks ago but hasn't started (0% Complete and 0 Actual Hours). The SPI=0 (correct).
But if I change the Start Date to today or greater (which is what a PM should do for tasks that didn't start on time) the SPI changes to 1, indicating the task is on time (it's really 2 weeks late already). It should be 0 until a % Complete is entered and then be calculated on the original start date.
It appears the SPI doesn't use the Baseline when calculating Planned Hours Scheduled to Date. It is using the current (new) Planned Hours Scheduled, which is technically incorrect.
From your Help Screen:
SPI = (Total Planned Hours x % Complete) / Planned Hours Scheduled to Date*