Interesting question and while no one admires a numeric answer any more than me, I think you will be disappointed by D/L. The numbers just won''t address your concern for "safety".
Here is an address for a very useful Excel spreadsheet full of statistics and calculated ratios for hundreds of boats. You can set your own thresholds and see the ordered results if you are at all Excel-facile.
select "Sail Boat Data Base"
A light boat can be perfectly capable of safe handling in heavy air. Js
do it all the time.
Now if your concern were comfort, weight can become a real virtue. Instead of D/L, look at the [Brewer] comfort factor, calculated as: COMFORT FACTOR = disp / (.65 * (.7 * lwl+.3 * loa) * beam^1.33). Results can vary from approx. 20 to 60, with the larger numbers representing a more comfortable (and almost always, slower) boat.