Start 3000 defensive specs and 299 solders.
Release 1 spec to soldier. Do not release the soldier.
Now 2999 defensive specs and 300 soldiers.
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It is also unlikely that it will be ever be done with 1 unit. Having something like 4000 specs and 200 soldiers is more likely. In that case 183 defensive specs released will give you total 383 soldiers and 3817 defensive specs. The math is quicker if you ignore the peasant change which always gives you a margin larger than rounding. If you release 200 specs to soldiers and it rounds up you desert 40 soldiers instead of 39 soldiers. Releasing 183 specs and deserting 39 soldiers hurts but it is much cheaper than deserting 400 specs and the left over soldiers can protect your elites and thieves.
Sending specs to the dragon to drop population and/or having kingdom mates send a soldier ball might be better than releasing if those are options.