How to decide optimisation objectives and element density threshold

I’m trying to optimise the geometry of my part for a project I’m working on. I would like to minimise the compliance and the mass of my part as much as possible given a stress limit. So I was thinking of making the objective of the optimisation to limit compliance x mass, however I’m not sure how to do this. Could someone advise how I could?

For now I’ve just been minimising the compliance and setting a target mass fraction. This has worked to produce a result but when I look at the result in hyper view I’m unsure what element density threshold I need to use to create a viable solution. If I put it too high then the stresses will be higher than allowed but if I set it too low then the weight will be higher than necessary. Could someone advise how I should decide which element density threshold I should use?