You add the rennet right at the start of the 24 hour period, not after 24 hours (at least, that's what I've always done). Try it that way (even if the book says it the other way round, that could be a mistake - a fair number of the books have some pretty brutal errors in them). - actually, just checked my notes, I've left it to ripen for a few hours before adding the rennet as well, but maybe only 6 or 8 hours, then added rennet for the last 16-18 hours.
As for metric/imperial, I grew up in Canada. Same thing with us, both were legal units for a while. I still think of fish in pounds, people's height in feet and inches, distances are in miles, beers in pints. Milk, though, I'm good in litres, so the conversion is happening!