The rule of thumb is 100IU per day, per ng/mL blood level. It depends on body weight (smaller people need less) but it's a good place to start. So say you wanted your levels at 40 ng/mL, you'd start with 4000IU/day. Then check in a month or two to see where you're at and adjust. If you go out in the sun a lot you wouldn't need to take as much either, or course. I'm guessing with a name like Heidi you're fair-skinned to some degree and won't have any trouble making some D3 endogenously as the summer warms up.