I'm sure many others will post this, but here's what I'll say. It'll get you close, but not all of the time. Lots will depend on your shooting form, consistency in your form, etc. You will also have to enter the information about your bow, arrow, with or without additionals, and string types exactly as they are to get things close.
I'll say this, sometimes it has been right on, and sometimes it's been WAY off. It is a great resource to use and it's fun to tinker and play with different set-up and then go check to see if they work for my set-up. Have fun, be patient and enjoy this great resource.