most decaI've seen (Organon, Hellas) comes in 200 mg vials, 100mg/ml.
give us your stats what are you stacking it with and we'll be able to help you better, but deca is normally used above 300-400 mg week to see any kind of effect.
And it being a very long acting ester, (decanoate) you can shoot it once a week and it will be fine. Of course, twice is better but barely significantly so.
Some people here hate deca, but to me it has been good, no deca dick, no hard shutdown. after a 10 week cycle, 200mg E4D stacked with 250 sus.