Ok, so the end goal I have is a chart showing the % changes from a start position over time from a starting position so:

Jan 1st    800    

Jan 2nd   837  +4.6%

Jan 3rd    783  -2.1%

Jan 4th    845  +5.6%

Jan 5th    847  +5.9%  ... and so forth

I could then plot these values cumulatively to show the %change from the original value over time

The problem is that the PREVGROUPVAL only allows you to look 12 steps back which is great for month by month summaries but I was wanting a lot more than that so that I could do a day by day movement and chart it on a rolling 90-day basis.  If these weren't percentages, you could just use the day-to-day change and plot them cumulatively but I'd rather show the percentages so that I can compare equally between people. That means I need to be able to reference the earliest row in the summary report

I was sort of thinking you could do it by seperating the first row off with a TRUE/FALSE field and then bringing the previous Parent function into the formula but I don't think you can do that sort of formula-based split in the reports (I can't see Date as a summary field to use in a formula).  I could set it to update as an APEX job schduled to run each day but I'd rather check whether I can do it in the report via formulas before I set off on that route unnecessarily?