Log initial size

Hi,I have set my db log intitial size as 512 MB and autogrowth as 512 MB. The db is in full recovery model and tlog runs every 15 min. Noticed, the very next day log size increased to almost 3 Gb. And my VLF's are almost 50 with only one row as status 2.I know shring the log reduce intitial size. But shrink is not good practise.Is there anyway to maintain fixed initial size. May I know the reason for Initial log size growth? If i ignore cant except how long initial log size grows.Thanks in advance.