Definitely test first as there is a huuuuge variation in what folks need. More body fat and/or more inflammation generally means more D is needed per pound - but not always. There are genetic differences in vitamin D binding protein that make it very hard to determine without testing.
Ranges for adults I know are from 2000-10,000 IU per day. O_O. Cannel from the vitamin D council says 5,000 IU per day for adults (though with qualifications and exceptions). One acquaintance who is very sun avoidant, yet very healthy and low body fat reached levels of 90 ng/mL taking 5,000 IU per day. He's 165 lbs. Cannel indicates 400 IU per 10 lbs body weight for kids (which is the same as 1000 IU per 25 lbs body weight) but again with qualifications.
Sufficient preformed A but definitely not excessive A is important as it increases D needs. I'm somewhere in between the Vitamin D Council (pro-D anti-A) and the WAPF (pro-extremely high A doses and high D doses). I think adults need 400 IU for every 10-25 lbs body weight (from all sources) depending on a variety of health issues and about 3x that in A per day.
In Michigan in the winter you are getting no D from sun. Nada. So starting in a few weeks, assume no D from sun (even if it's hot, even if you get a burn....) and test/dose accordingly.
grassrootshealth.net has a great home D test that's part of a population study. It's a very accurate test and is a great project. Do consider joining if it's at all a possibility.
Oil based D is good - not dry tabs. Anything from harvest health is fine. I happen to like iherb. Runs about 40% less than Harvest Health. Here are some google docs I put together to cover what supplements I use, where I buy and more. http://goo.gl/KmsCx
yes, feel free to repost/share. add the qualification that I realize there is always new information coming out and that recommendations will change. And that even among the vitamin D scientists there is considerable variation in recommendations for amounts, though typically they agree that an ideal serum level is 40-60 ng/mL (100-150 nmol/L). This recent information may change that guideline to 48-60 ng/mL (120-150 nmol/L). The 40-60 ng/mL guideline comes from grassrootshealth. This project has all of the major D researchers *worldwide* on board. Their info trumps Cannell, WAPF and Jaminet for the general population. For some specific instances, Jaminet (on the low end) and Cannell (on the high end) probably have some valid points and make recommendations worth considering.