Alternator issue I think

Just wondering if anyone could shed some light on my issue.I am running the Nova Gsi 6 Dial clocks in C20xe when I first start the car the volt gauge seems to sit low I have to rev the car up in high rpm an the gauge shoots up to halfway. I did a test on my battery and it seems to be getting a charge of 12.57. Also I thought I would mention when driving and using my indicators an lights the volt gauge needle will go along with the indicator or flash my head lights an needle will move I am thinking it is more towards a knackered alternator but any help would be appreciated also my engine management light has come on now and seems to splutter on low revs