You can change the default units for your account by going up to Account > Profile > Use Metric Units
This will change all of your numbers to metric.
A lot goes into getting trees to render properly. Flying higher helps. Flying on windless days helps. But more overlap is the best things you can do. Rendering trees is why we added the 80 and 90% overlap options. The standard setting is 70%.
Also, we generally recommend flying at least 4 times higher than the tallest thing you are trying to resolve which can put you up against the DJI imposed hard deck if you are trying to map anything taller than 30m.