Particularly, if you have a start-up grant in mind, with upper budget limits, calculate what you can do with the limits you have. In fact, the two problems often go together. Lack of focus frequently results from having too many general-sounding aims, i.e., being ambitious without being specific. Most of us have difficulty recognizing when our writing is unfocused. Read other successful applications submitted to the same grant maker, and if possible the written critiques of those successful grants, to see what the reviewers are looking for.
Nothing persuades a reviewer more effectively.
Beginning investigators often see this as a catch-22. Apply for some small start-up grants that may not require pilot data. This will allow you both to generate data and to show that you can get funds, before you apply for a larger grant. When you are negotiating for your first job, ask for the start-up funds you truly need. Once your new employer has chosen you, they want to give you reasonable resources to succeed.