Compromise - Half (or thereabouts)

In the Duel of Wits the rules for a compromise say that you get a regular compromise if your opponent’s Body of Argument is reduced by half (or thereabouts). So we were running a test last night and this came up. The Evil Wizard from BWG vs the Idealistic Young Priest from Adventure Burner. The wizard won, but lost 4 of his 10 BoA. That’s only one point away from half, but it’s also 10% away from half. Unless I’m mistaken BoAs won’t get very much higher than that (20 max at high level play). So, how many points away from half is “thereabouts”?