Critiquing tremolo technique

I test my tremolo ability on one string at 4 different tempos. I used the forum to get feed back and make adjustments which have allowed me to pick faster though I have trouble keeping it steady and on beat. I try to keep the pick perpendicular with the line I cut through the string but that line seems pretty steep so I don't know if I'll be able to translate this approach to strumming multiple strings. Any advice on how I can improve my technique is appreciated.