it looks like you have already done the calculations. The instructions give you the amounts of components to add to make 50 ul and you want 25 ul. I can see that you have written down half the amount of all the other components and have come up with about 17 ul, so just add 8 ul of water and that will give you the final volume of 25 ul. This mixture will have the correct amount of enzyme concentration, as well as the correct concentration of the other chemicals needed for the reaction.