What’s the speed for lowest sink rate on a DA20?
Admittedly, my prop performance theory is a tad rusty, so maybe someone else can provide proof here.

I seem to recall that - assuming a quadratic drag polar which might not hold true for your aircraft - minimum sink speed has a theoretical value of .76 x best glide speed, so in your case that would be around 56 KIAS.

If you’re comfortable and consider it safe to do so, why don’t you (at a safe training altitude) trim for 73 KIAS and establish a stable idle descent, note vertical speed descending through a predetermined altitude, reposition to initial altitude and repeat in maybe 5kts decrements down to stall speed or some safe margin above - the former of course only if flying right down to stall is still trained and allowed in small aircraft. This will give you at least a rough idea of where minimum sink speed lies and what rates to expect.

The DA 20 Approved Flight Manual doesn’t help much, by the way.