I have a 13 week old baby bun and instead of giving him unlimited pellets all at once, I feed him twice a day, but give him pretty much as much as he can eat. So he gets 1/4 cup in the morning and it’s usually all gone when I get home from work, so I give him another 1/4 cup. If it’s almost all gone when I’m ready for bed, I give him a little bit more (not a full 1/4 cup). Now that he’s started on veggies he doesn’t seem to eat quite as many pellets, so he doesn’t really need the extra bit at night. Doing it this way allows me to keep track of how much he’s eating and whether or not he’s eating, which I like. Considering how much Otto eats, 1/8 cup does sound like too little for Parsnip; I’d just go with the HRS guidelines and give her as much as she can eat for now.
ETA: I feed Oxbox 15/23, which is alfalfa. For hay I give alfalfa sometimes and timothy others so that he doesn’t get stuck on eating just alfalfa.