Anyone who thinks 50nmol/l us a "healthy" 25(OH)D level hasn't been paying attention to the recent research.
The NATURAL level for humans living as human DNA evolved (near naked outdoor lives in East Africa) is around 125nmol/l or 50ng/ml at which level human milk is a vitamin d replete food for human babies. At 50ng/ml 125nmol/l the Vitamin exerts it's most potent anti inflammatory action.
When people are obese the fat cells produce pro-inflammatory cytokines and the body uses up Vitamin D resolving that inflammation hence 25(OH)D are always lower in obese/overweight people.
It makes sense to relate Vitamin D intake to body weight so bigger bodies get more vitamin d, One size doesn't fit all with vitamin d.
Whatever daily intake you suggest there will be a 100ng/ml or 250nmol/l range in the response, from the fattest to the thinnest person taking that daily amount.
So the only sensible suggestion is to a get 25(OH)D tested regularly (every 3~6 months) until such time as it takes you to work out how much vitamin d your body requires to stay around the level 50~60ng/ml 125~150nmol/l we know is natural for human DNA.
If you ran your car with oii/water/air/or brake fluid levels/pressures 25% ~50% of the makers orignal specifications you would spend more time dealing with problems at the garage. Keeping the population with Vitamin D levels less than half that human DNA evolved to attain and maintain provides a lot of extra work for health professionals and those who depend on the health industry for their profits. Hence the reluctance to see the obvious connection between low vitamin d status and health problems.
City Assays Vitamin D Blood Spot Test �25 UK �30 overseas
Grassrootshealth also do Vitamin D blood spot testing
there is a Vitamin D measurement converter on their website.