Here's an approach to the rounding up function. I've left the algorithm of what to round by at each level as exercise for the reader, as you said you know how using log()

CODE:

#!/usr/bin/perl -w
use strict;

# $n is number to round, $d is "divisor" for rounding units
sub roundup {
        my ($n, $d) = @_;
        return ( int( $n / $d ) + ( ( $n % $d ) ? 1 : 0 ) ) * $d;
}

for my $i ( 0,4,5,6,9,10,11,24,25,26) {
        print "$i -> " . roundup($i,5) . "\n";
}

for my $i ( 99,100,101,124,125,126,149,150,151) {
        print "$i -> " . roundup($i,25) . "\n";
}

for my $i ( 999, 1000, 1001, 1249, 1250, 1251, 1499,1500,1501) {
        print "$i -> " . roundup($i,250) . "\n";
}

RESULT:

0 -> 0
4 -> 5
5 -> 5
6 -> 10
9 -> 10
10 -> 10
11 -> 15
24 -> 25
25 -> 25
26 -> 30
99 -> 100
100 -> 100
101 -> 125
124 -> 125
125 -> 125
126 -> 150
149 -> 150
150 -> 150
151 -> 175
999 -> 1000
1000 -> 1000
1001 -> 1250
1249 -> 1250
1250 -> 1250
1251 -> 1500
1499 -> 1500
1500 -> 1500
1501 -> 1750
[download]

-xdg

Addendum: This rounds up, but not over -- for the case where 4 rounds up to 5, but 5 rounds up to 10 (!), then the function is even simpler:

sub roundup {
        my ($n, $d) = @_;
        return ( int( $n / $d ) + 1 ) * $d;
}
[download]

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