Your maths is correct assuming the mortar and target are at the same altitude.
Handy little calculator here for future reference:
http://www.convertalot.com/ballistic_trajectory_calculator.htmlAcceleration due to gravity varies depending upon altitude, varying from 9.78 to 9.82ms
-2 on earth. Don't worry about it, just use 9.81.
Drag is quite difficult to model correctly because of varying air pressure, temperature, humidity. Try putting it in, but set your mortar shell config "airFriction" to zero for tests without drag.
Don't forget to add an aiming error coefficient to give the rounds some inaccuracy - I'm presuming in rl a mortar strike isn't bang on the same spot from the word go and there will be a spread.