When Anna used Colistin it was the 150 twice a day as well. She mixed with 2ml sterile water. Roll the bottle, don't shake it to minimize the foaming. Only mix one day at a time.
I'll repeat what so many others have said, check with your clinic on the exact prescription. If you originally were going to get the script filled with CFF pharmacy you could even call them and ask them what the script was written for.
Although it might be time consuming, can you appeal to your insurance co. for neccessity of the prescription? For the next time. I believe that the pharmacutical co. also has a program to help get the medication.