The 25g amount was (somehow) arrived at being the "best" amount a normal sized person should take to provide the best chance at a successful ctb, EVEN WHEN PUKING IS TAKEN INTO CONSIDERATION. The way I understand things is the body views SN as a poison in the amounts we're talking about, even significantly lesser amounts than the 25g. It's going to take steps to remove even 10g. The idea of 25g is to try and get more into your system before vomiting. If you try and take less, you (probably) won't vomit quite as fast, but who knows? But, you stand a greater chance of not getting enough into your system and having a failed attempt. If you try an amount greater than 25g, it will (probably) just result in a quicker vomit reflex, maybe even more of a violent one, but I don't really know for sure.
I'd just stick with the 25g/50ml ratio and have a couple/few cups mixed up in case you have a very quick vomit reflex. Honestly, after puking, you're going to have a hard time even drinking one extra cupful, let alone multiple ones. Having enough for 3 or 4 cups won't hurt anything I guess.
Also, you could try an over-the-counter medicine with AE properties called Dramamine, but it most likely won't be as good as a prescription one.