It's 70% of THAT tough boosted part. That's why the tough parts are always first. This means that fully boosted tough parts have an EFFECTIVE hit points of 330 hit points. There is also a "stacking effect" between tough parts and heal parts, as heal parts heal base hit points. So one tough part requires 330 hits to disable, but only 100 heal to recover fully.
Short story: At level 6, you have 2 towers. That's a total maximum damage of 1200 hits. A T3 (Tier 3, the highest tier.) of boosted attacker needs 4 tough parts (1320 hits for those 4 parts.) and 9 heal parts ( 432 total healing) so it can fully recover all 4 tough parts in all cases.
TL:DR. T3 boosted creeps will steamroll a level 6 room.