The entered Stress Goal is the target of the optimisation. At the end of the optimisation all material should be evenly loaded with this value.
The occuring stress highly depends on the applied loads. In most cases the yield strength of the material is not a suitable value. If the Stress goal is assumed to high and the loads aren’t able to create such high stresses, all the material will be reduced.
For the Stress Goal an educated guess should be done. If you have no clue which value is a good starting point for your model, just start with a low value. Have a look at the approximated stresses in your first iterations, terminate the simulation and modify the value accoridingly. The calculation of the first iterations should only take a few minutes. The Stress value can also be found in the analyse.log file in the optimisation result folder.