Help with First Build

Been gone a while, I have all the parts I need, but I’ve been busy with school starting back up. Anyways, I started piecing together the motor mount, motor, wheel pulley etc. I’ve found the belts I have are too long. How would I go about calculating a belt length for use with an idler pulley. I’m using Boardnamic’s motor mount. I KNOW there is a belt length calculator but it cant calculate for an idler pulley, and please don’t just tell me to add a few mm and that worse case belts are cheap. For one, I’m not one to waste money when I don’t necessarily have to, and I’m also a college student on a budget. I’m using orangatang kegels and the lower bolt for the hanger clamp is about 2 mm off the ground and it hits when just riding my board around with the clamp on. I’m going to have to figure out something for that too. Thanks guys!