How to know when to increase formula?

Probably a stupid question. So guidelines tend to say 150ml to 200ml per KG of baby's weight in 24 hours. How do you know when to increase it when you aren't given the opportunity to weigh the baby regularly? So once baby is over birth weight, they aren't weighed by HV or GP very often. So I know what he was when he was last weighed a few weeks ago, and I make sure he gets at least 150ml x that weight in 24 hours, and any more he wants. But he's presumably put on weight, so without going to a weigh in regularly, how do you know when to increase the daily total/aim for a higher daily total? I know to feed on demand but I want to make sure he's getting enough!