Can I open and weigh out dosage with a digital scale? Cephalexin capsules 500mg. Need 250mg every 12 hours!

My dog has a UTI and needs antibiotics. I've owned her a very long time and I know exactly what is going on ( please know that I know this animal very well. My friend was able to get me cifloxacin from her vet today however they went by the weight of her dog who is 80 lb. My dog is about 35. Am I able to open up the capsules, weigh them out, and give my dog half the capsule as long as the milligram is correct? I read in a few different threads and websites that this is okay just frowned upon in case people do not have a consistent and accurate way to make sure the medication is accurately measured.