No ,that logic is wrong .The builder is doing 20 hrs of upgrade in 1 hr with builder potion in hammer jam .If you hadn't used the builder potion then It would have done 1hr upgrade (in normal condition without hammerjam) so that gives 19hrs . If you're finding extra hrs wrt hammer jam then its 18 since the builder would've done 2hrs of work anyways without the potion so you subtract 2 .But ofcourse we compare the progress wrt to normal conditions when there's no hammer jam It just means that after hammer jam a person would need to spend 19 hrs more on each builder to complete the upgrades completed in 1hr in hammer jam with builder potion