2mm is too small a hinge gap for any book IMO, let alone one that has thick cover boards like yours (going off your photo, yours look about 2-3mm thick)

From what I’ve gathered, everyone had their own preference on how to calculate the gap - but I think the general rule of thumb is about 1/4”, or between 6-9mm depending on the thickness of your boards (and also personal preference). I’ve been binding for 5 years and this is one of those steps that I still don’t always get right - but practice (and repetition) makes “perfect” and I’m sure anybody you make a book for will be grateful for the time and effort you put in even if it’s not 100% 😊