preparation of any media take place according to instructions. So if the instructions imply that 38 gm in liter so for 100 ml add 3.8 gm. Or if you mixed so take 0.1 of the quantity.
Weigh and pour required amount of MH medium powder in a 250 ml beaker containing 90 ml distilled water. Adjust it's pH to 7.3-7.4, make the final volume to 100 ml, transfer whole content into a conical (250 ml) capacity flask, and autoclave to make it sterile.
Simply follow Manufacturers' instructions to prepare any medium. It is frequently written on the container. If you have prepared 1 L of the medium already and you do not want to use all, then use 100 mL from there and discard the rest. It's that simple. I don't see any point in trying to derive 100 mL of medium from what you have already prepared. Just use the volume you desire.
I second the opinion of Maurice. Though majorly, the media preparations are the same for all brands, at times, I have observed minor differences in the preparation techniques, which vary among manufacturers.