Points are determined by the performance of a given machine relative to a benchmark machine, similar to the CPU client benchmark process. Before releasing any new project (series of work units), we benchmark it on a dedicated computer with an ATI Radeon 3850 GPU (512 MB, 320 Stream Processors), running in a Dell Inspiron 531, with a 2.16 GHz dual core AMD 64 X2 4000+.
We plug the results of this benchmark test into the following formula:
Points = 1500 * (DaysPerWU)
where DaysPerWU is the number of days it took the benchmark hardware to complete the work unit. Note that the GPU client still relies on a fast CPU, so the CPU is an important part of this. The Points Per Day (PPD) given here assumes that a CPU is heavily needed, with a larger PPD to compensate for the use of that CPU.
Please note the very concept of a reference machine will mean that some WU performance will vary from the performance on your machine. Even between various GPU models, there are significant differences in architectures and memory speeds. Moreover, there are variations between WUs within a given project which can lead to speed differences.
Our goal is consistency within a given definition of a reference machine setup (described above), but beyond that, the natural variation from machine to machine and WU to WU will never allow any point system to perfectly predict what you get on your machine.