Refresh rate in DDR-SDRAM

HI all,
I wanted to know about how refersh rate is calculated.Average
Refresh rate for 256Mb is 7.8u & for 128 Mb it is 15 us.I read in a
doc that its 64ms/No.of rows.I read in a document that 8 auto-refresh
commands can be posted to SDRAM to improve scheduling of tasks.
There are two timing parameters
1.Refresh to refresh command interval
2.Average periodic refresh interval.

How are they related.

Please somebody clear my doubt..

Thanks in advance,
Raghavnedra.S

Raghavendra