Cognos 8 Report studio SQL problem

The project is working out the Average Net Selling Price. This is Grouped
by Period and Average Net Selling Price(ANSP), and then working out the
running total % of volume(KG) by ANSP. You then proportion the ANSP in to
case statement buckets and work out the minimum ANSP for that bucket. You
then work out the spread from those minimum ANSP. which is charted.
Rules
This data is then case statement into buckets
SpreadGroup ANSPGroup
0-5% 0
5-10% 5
10-20% 10
20-35% 20
35-65% 35
65-80% 65
80-90% 80
90-95% 90
95-100%

All data is held in one table.
There is no lower grain to this data
When 5% is rule is broken then first ANSP is used
When the jump occurs say at 20% then this ANSP valvue should be used for
previous 10% and 5% and thus say 0% as the starting point of the chart.
All buckets must exist even if the volume % jumps > 5%

Considerations
Its not possible to lower the detail/grain of the data
This has been created in Excel using macros and pivot tables, though Excel is not a enterprise reporting solution.
I belive this can be sorted in SQL but am lost on how to do so.