You should record a timestamp on when the person became MQL. Then use this timestamp to select the MQLs generated during a particular time period and measure how many of those became SAL.
It depends on how your lead life cycle process is set up. Our marketing and inside sales team work a lead until it is ready to become an MQL and then we convert the lead to a none forecastable marketing opportunity which is handed to sales to work. The sales team can then change the stage of the opportunity which triggers SAL, SQL etc.
We currently use Marketo to manage the lead life cycle based on updates made within SFDC, this allows leads to move through the stages of the waterfall and track when things move and to what stage.
In order to track we use reports that look at opportunity created date e.g MQL and then we use historical opp reports that look at when the stage of the opportunity changes, therefore we can track how many MQL's we created and then when they moved through SAL, SQL, Won, Lost or Return to marketing. We always have an accurate picture of when an MQL was created based on created date and when the stage was changed irreverent of the quarter.
Hope this helps, give me a shout if you have any more questions,.