Disk Read Latancy and Disk write latency calculation

Hi,

I'm using this wmi query "
SELECT * FROM Win32_PerfRawData_PerfDisk_LogicalDisk WHERE Name!='_Total'" with namsepace asroot\cimv2 to get the disk related metrics but please let me know from this how to calculate the disk read latency and disk write latency metrics. I need an formula to calculate the latency.

Thanks in advance!!

Regards,
Raamesh Keerthi N J