I take it that you have the two documentation sheets that came with the vials?
As an example; one recommended development time for CHS 25 is 7 minutes in 500 ml of solution@20C. The solution is created by emptying the vial in a container and then topping up with water to 500 ml.
You want to use 300 ml of solution. One way of doing this is to empty the vial in a container and topping up with water to 300 ml. Correct developing time for 300 ml is to multiply the suggested 7 minutes (420 seconds) for development in 500 ml of solution with 0.6 which gives you a new development time of 4 minutes and 12 seconds (252 seconds).
4 minutes is a bit short and may result in uneven development. Another way to do this is empty half a bottle in the container, top up with water to 300 ml, and multiply the suggested development time for 1 vial - 500 ml of solution with 1.3. That is 7 minutes x 1.3 = 9 minutes and 6 seconds.