[Sebboh]

I guess this is more of an undergrad problem which is why I posted it here. 
I often work with solutions and have to obtain a certain osmolarity so I need to add a specific volume of water if the osmolarity is too high. This can be a chore because the osmometer takes a while to read the sol and I don't want to go under the desired amount so I often have to add little bits of water until I get the right number.
I think this equation I use to calculate how much water to add is correct, but I'm not positive and I would also like to know how to derive it or how to derive a better one. (I've tried it and it works well enough, but I don't trust it yet.)

(#osmoles off your desired amount/#osmoles desired) x amount of solution

For example, if I have a 200mL solution with an osmolarity of 350 and want the osmolarity to be 300 and would like to know how much water to add, it would be: 50/300 x 200mL = 37mL.

Although that seems like quite a bit.

If anyone could help, I appreciate it.