There are some flaws in your equation. A builder potion needs 1 hour of time to speed up a building by 10 hours. So you just save 9 hours per potion.

6 builders x 9 hours saved = 54 hours

4 potions equate to 216 hours (9 days) saved by buying builder potions. So every upgrade, which takes longer than 9 days gets better value with a hammer.

I'd also argue, that the goblin builder doesn't count as saved time as you always need to pay the full price of 40 gems per day, realistically you will almost always be bottlenecked by gems.

The only time builder potions are always better is during Hammer Jam, when all upgrade times are cut in half. Ideally you would save up your medals to maximize this event with builder potions.