You already use sprintf for formatting. Why don't you develop a full line template for your report like this;

# header
printf "%4s %20s %20s %20s\n\n", "Year", "Balance", "Interest", "New B
+alance";
....
# report (in while)
printf "%4d %20.2f %20.2f %20.2f\n", $year, $StartAmountRounded,$Inter
+estrounded, $NewBalancerounded;
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Of course, the header can be done directly, using printf there is just for demonstration purposes.

Also, if you would provide a little standalone running script, preferably with use strict; and use warnings; you would get more answers to your problem as it is far easier to run a script and look at its output than to read the code only.