I have a different method for tremolo: I do it p m i a
If we look at the usual sequence, p a m i, we noticed that weakest link is perhaps the part which we play a finger followed by m finger. Due to the way the fingers are structured, we are likely to end up playing:
(Notice the gap between the a and m is shorter)
I had slowed down recording of my tremolo using this method using computer audio software and this becomes very obvious.
Another experiment to confirm the weakest link is to do a scale with im, ma, and ia we would find that ma is the hardest to do. So by re-arranging the sequence p-m-i-a, we replace the weakest part of the link (m,a) with i, m finger which is much more independent. Thus we are able to achieve a smoother tremolo. However changing the order took me a few month of getting used to, since we are so accustomed to the p-a-m-i order.
With this I would like to deduce that if we are able to do a scale with m and a almost as fast as we can with the i and m finger, then we would be able to achieve a smoother tremolo. Just a thought.