I offered my DS1 the occasional formula feed from around 3mo and I'm pretty sure I just used the recommendations on the formula and followed my DS1s lead as to if he finished it or not.
Generally speaking if they are getting enough they stop feeding and will leave a bit in the bottle.
Formula takes longer to digest than BM so you may get longer between feeds,but despite what books lead us to believe, there isn't a one size fits all and it's mostly part of the lifelong parenting journey, that you need to make the best decision you can at the time using the information that you know about your child. What works perfectly for one child can be disastrous for another, so it's important to be looking to what the needs of your child specifically rather than thinking what is 'right' because in the 1960s 'right' was canned milk with sugar added...which now has us all horrified.