Let's start with I need a multimeter. Brains trust, what is a good buy? Is this worth it? I want peak voltage, too, is that what "hold" function does? https://www.amanstoyshop.com.au/productdetail.aspx?id=860
Might be more that you want to spend but Fluke meters are about as good as they come. There are quite a few in that price range... this might help explaining about Peak Voltage https://www.fluke.com/en-au/learn/b...-measure-duty-cycle-with-a-digital-multimeter
When checking duty cycle (as in above ) to test coils or injectors the hold button will keep the reading. Otherwise the reading will just go up and down with on/off time of the current to the injector/coil. Why test duty cycle? To find out if your injectors are getting to their limit, eg if running more than 90% at max revs/boost then you need to replace them with bigger injector bodies to flow more fuel.