Dosage is very difficult to determine unless u get a 25 hydroxy D test every 3 months until u figure out at what dose u get into the optimum of 60-80 ng/mL.
Here are some graphs showing how much D to take to get to a level if you are at a level and other graphs.
I came up with a measure I use which is 45 iu per pound of body weight as a daily maintenance dose, but have not been able to test it in more than one person, yet.