@ISmolin having those date properties doesn’t change anything here unfortunately. Conversion rate reports only work with HubSpot’s own stages and phases. You would have to use lifecycle stages to achieve this particular report. If you stick with your custom properties, you would have to report on this outside if HubSpot.
Edit: Nevermind, actually, I overlooked something simple. Create a pie chart of all contacts who have have ever had your SQL flag. Break it down by contacts who have your SAL flag. You will now see a slice of contacts that is the % of SQLs that have gotten the SAL flag. Filter the report by your SQL or SAL date to limit the date range.