So in the 1.5 months of the year that you don't get the stipend you are getting active duty pay for an O1 officer (3107 a month and since you are active duty for 45 days you get 3107 x 1.5 = 4600 roughly). You will also get BAH during this 45 days, which is based upon where your school is. You can use a calculator
BAH Calculator to figure out how much O1 get paid in your zip code for that time looks like it can be lower than what I said before. Mine in my zip code is 1083 a month so (1083 x 1.5 = $1624). Last, Basic Allowance for Subsistence (BAS) is used to pay for food for Enlisted Soldiers and Officers. This is a flat rate no matter where you live and for officers it is 254.39 a month (254.39 x 1.5 = $381). So for that 1.5 months of "active duty" every year I get paid $6605 before taxes over those 45 days or (6605/1.5= $4403 a month). So for the 45 days you get paid much more than the stipend is, but this is only during those days. The rest of the 10.5 months you will get paid right around 2,000 a month after taxes. All in all it is much more money than many medical students live off of each year.