You used 30ml of bacteriostatic water for 5000iu.
5000iu/30ml = 166,67iu per 1ml.
250iu is the amount you want to inject in 1 shot.
250iu/166,67ml = 1,5ml of bacteriostatic.
So yes, you are correct.
You might want to use 1 or 2ml per 1000iu in the future. So you would need between 0,25-0,5ml for 250iu. I would aim for 0,25ml if you're using a slin pin. Should be easier to inject less ml if you're injecting subcutaneous.