The correct dosis of baytril is 0.4 ml twice a day. Your vet is playing it far too safe, I'm sorry to say!
If you can badger your vet into prescribing pediatric septrin 0.5 ml twice daily, if your symptoms don't go away, that would be a fight well worth. I've done it with my vets this spring. Septrin is a mild antibiotic for children and has a banana flavour - my guinea pigs LOVE it and it is a lot less hard on the guts as baytril! The problem is that baytril is the only listed antibiotic for guinea pigs, but septrin works better for UTI's.
I hope that your vet has given you some painkiller (for example meloxidyl for cats, ca. 0.2 ml once daily for a 1kg piggy) and some probiotic to counter the effect of the baytril on the guts. You can get probiotic from p@h if necessary; it will make Buttercup feel better in herself.
MAKE SURE THAT YOU HAVE AN X-RAY without anesthesia! The feeling method, apart from being inconvenient for your piggie, is not at all conclusive, and if there is blood in the urine, you really need to check for crystals and stones.
You can do a little home check, by placing Buttercup on a plastic surface and leaving her there until she's had several pees. If the pee feels gritty, then there is indication for crystals!
I have lost one of my guinea pigs to bladder stones/cystitis; and had two more this spring with infectious cystitis, but I made certainly sure with both that there was no other bladder trouble!
Make sure that Buttercup gets as much fluid (cranberry juic and water) as often as you can manage to flush out the infection.
All the best to the poorly patient!