sure...
100mg/ml means that each ml has 100mg of active ingredient in it. That means 2ml = 2 x 100mg = 200mg total of deca if use the entire contents of the amp. If you need 400mg/week of deca, you'll need to do 2 amps or 4ml total.
Since you're new, I'll first say WELCOME! And second, I'll ask what else are you stacking the deca with and what are your goals? I believe strongly that if you're going to inject yourself with androgenic/anabolic oils, you should know as much about what you are taking as possible. That means planning a cycle based on the result you want and stocking everything you need including anti-e's and post cycle recovery goodies ahead of time. It also means researching the gear you're thinking of working with so you know what to expect as far as sides go, and have some counter measures planned. Sorry to sound preachy...just looking out for anyone who cares enough to join here and try to better themselves.