I would do it based on how many iu's you want to do per day, i.e. how many shots you plan to get out of each 18iu's.
If you want to get 5 shots from 18 iu's, use 1 cc of water. Then each 1/5 (.2cc) is 1/5th of 18iu's, or 3.6iu. So take .2cc to get 3.6iu.
To get 4 shots from 18 iu's, use .8 cc of water. Then each 1/4 (.2cc) is 1/4th of 18iu's, or 4.5iu. So take .2cc to get 4.5iu.
To get 3 shots from 18 iu's, use .9 cc of water. Then each 1/3 (.3cc) is 1/3rd of 18iu's, or 6iu. So take .3cc to get 6iu.
To get 2 shots from 18 iu's, use 1 cc of water. Then each 1/2 (.5cc) is 1/2 of 18iu's, or 9iu. So take .5cc to get 9iu.