when a calf is born I log it. Use the alphabet letter for the year. First born this year is F01, second F02 and so forth. When I get around to tagging, I use a pre printed calf tag. I don't worry about a specific number, I just tag it. I will reconcile that tag with it's birth order and mom at a later date as if 01 was tagged with tag number F15 I will alter the calf's I'd number to F01-15 on the records. If I were to keep a heifer for replacements at some point I'd give it a cow tag. Number it with the alphabet I'd for the year it was born and use it's original order of birth number. So if I kept F01-15 for a replacement, her cow tag would be F01.
Cows I buy keep the tag number the have when I buy them. I'll give them a couple initials representing where they came from. If they were bought through auction and not ear tagged I'll use the livestock commission number for their I'd.
Sounds complicated, but it isn't.