I think writing SOPs are a very hard thing to do. No only do you not know when it is too detailed or not detailed enough, as the author its hard to put yourself in the shoes of someone who knows nothing about your experiment and needs steps. With that being said too much detail isn’t always a good thing as you don’t want a 30 page SOP as well. It is best to just have plenty of people review it or try to execute the SOP to see where it isn’t clear and go from there.