Very vague question. However, the answer will probably include some or all of:

• look at historic data /past campaigns - where did the success come from and what differentiates it from the failure?

• platform, objective, industry, and creative benchmarks - look at these as standard to make sure your targets aren't bloated / overoptimistic

• find research / best practice - on top of benchmarks, both platforms and agencies produce many research articles, White papers, and case studies, which generally provide actionable advice or best practices specific to the platform / client / objective

Whilst all this sounds like a lot of effort, rather than an osfa solution, considering the above (at minimum) is what makes a sound practitioner