select (SELECT cntr_value
FROM master..sysperfinfo
WHERE (instance_name = 'YourdateBaseName') AND (counter_name = 'Log File(s) Size (KB) '))-
(SELECT cntr_value
FROM master..sysperfinfo
WHERE (instance_name = 'YourdateBaseName') AND (counter_name = 'Log File(s) Used Size (KB) '))