I think 10 gallons to 250Mg is a good ratio. Less water and the water can get too polluted too quickly, especially with larger specimens. Too much water and it just becomes a PITA to do water changes. I still opt for 100% water changes, because I feel that this removes as much of the pathogen as possible. Partial water changes inevitably leaves the pathogen in the tank.
One change I've made in process is that I wait a little while after the water change before I add the medication. As Nemguy mentioned, this give the nems a little rest period without any meds. During this period I watch to see if the nem has expelled anything.
One final thing to note -- if the nem is deflated and doesn't quickly inflate, it's usually a death sentence. I think the reason is that it can't absorb the medication if it's not inflated.