its all just simple math people. Take the number of units, in your case 5000 and divide that by the number of ML of BAW you add to get final concentration.
5000 / 5ml water = 1000 so thats 1000 units per ml of water.
If your syringe holds 1ml then how full do you need it to get 500 units? if 1ml = 1000 then it would be half a 1cc/1ml syringe to get 500 units. If your pin has marks on it all the better.