The Pletscher twin leg kick stand has marks to cut the legs to required length according to wheelsize or personal preferences. No thumb rule here as the OD of wheel depends on tyre size so better do an initial cut, mount the kickstand again on the bike and check (on a flat surface) how much the front wheel lifts from the ground. If still too much, then proceed to repeat the process until you get to the required height. Metal saw would suffice to cut the aluminium legs (round the edges of cut legs with a file). Use the plastic end caps for this twin model which (surprisingly) are not included with the kickstand.
As @goosenoose suggests a front stabilizer helps a lot to prevent a flopping front wheel.
I will say leave no more than 5cm from front wheel to the ground as it will help to keep bike stable when you are on an uneven surface (mostly all the time) and you will have a good chance of both wheels touching the ground. But as I said is a matter of personal preferences.