Just a thought but since you want to use the remainder for another batch, maybe you should do a 500 ml starter then you can shake it up and add 100 ml (before fermentation) of the starter to the beer you have and let the rest ferment out. It would be much easier to accurately measure 1/5th of a pack as a dilution than to try to get an accurate measure on a smaller amount.
Then you can let the rest ferment out, decant and top off with boiled and cooled water to save your remaining cake. Yes, much more work than the rest have recommended. My thought is simply this. If you won't brew again until your 1 gal batch is done, use the whole pack and then use that yeast cake for the next batch. If you want to brew again before the batch is done and want to hold the yeast in the interim, this could work.