Lots of ways of working out track miles and lots of variables, and therefore lots of ways of making a potential mistake.
Variables include theterrain, weather, duty runway, time of day (how busy is it going to be) which all may mean that you will have extended vectors for an instrument approach or a straight in visual.
The safest way is to use the FMS or map to calculate the shortest route onto the rwy via the SID and transition to calculate TOD, and then modify RoD as required.