Boil Level for Anchovy-Kelp Stock

Hello!

I recently made the Anchovy-Kelp stock from your cookbook, and, potentially confused by the language, accidentally kept it at a hard boil… for an hour and a half… :'(. So, I ended up with only a quart of stock from the whole recipe. That quart is utterly delicious, but I planned on having three quarts. In the future, should I keep the stock going at a gentle boil, or even a simmer for the hour and a half cook time? Every bone in my body was yelling at me while keeping a stock at a hard boil, so I definitely feel like I messed it up XD. Thank you!