When you are creating a goal, you have the option to choose how the progress will be measured.
Here are the different options:
Percent
Value
Currency
Completed / Not completed
Roll-up from subgoal progress
Roll-up from task progress
When you choose "subgoal progress" as the way to measure the progress of the goal, you will not be able to specify a start and end value as the progress will be determined by the subgoals' progress.
When you add more than one subgoal to a goal, you can change the weights of these subgoals and determine their relative impact on the goal's overall progress.