I didn’t know this, in fact I am one of those that put way too much tension on my best.
How much is too much you’re asking me? Imagine one foot on the truck while one hand is pulling on the motor as I tighten the motor in place! 
After watching this, I feel like a complete noob! There are some good points in there I didn’t consider too.
Loose as I can without slipping. Can depend on pulleys and motor mount length however
You want it as loose as possible, without it skipping when you slam on the brakes really hard from a medium speed. Just tight enough to not skip then.
the decel reason is a bit of a stretch. 
I just got done losing my timing-belt with enough sog (I just made this up ) and def no slipping. I kind of did a pinch test on both side to make sure they are equally tension.
Also having less tension on the motor will definitely help extend its life.
I’ve had a lot of slippage during breaking. I guess it comes down to priorities. I like to go fast but I’m usually near intersections and people, so reliable and effective breaking is more important than range or coasting.
I have this motor mount for a board with a single motor and no matter how tense I put the belt when I brake hard it always skips. I have given up and I’m building a gear drive board now.
Got a photo?
It looks too loose. Do you have a photo from the top down?
When you turn the wheel by hand forward, and turn it by hand backward, does the belt track to opposite sides? How far?
Your motor pulley is small compared to your wheel pulley. So you could try a longer mount and belt which would change the angle and engage more teeth. But an idler pulley would probably be better. This is also a reason to go to two motors. You double the surface area for breaking. Does your motor get hot. Looks like a lot of mads for just one.
So, I think I know the theory…belt should be tight but not too tight, loose but not to lose 
That should be easy in theory but in practice….
I have almost destroyed 2 belts in just a few km. Granted none of them has snapped yet. But they def. elongated and show serious signs of wear IMHO. I have a single drive (flipsky 6374) and know there are limitations and I’m seriously thinking of getting a dual esc and two motors. Yeah …I get to spent more money! :D. But in the meantime…What can I do? I tried Dickyho’s idler and destroyed one of the bearings right away, they are a bad design IMHO. The bearing used is small and has too much play in the pulley, and it’s made of really bad aluminum, my belt was silver after only a few km. Is belt tension with the idler supposed to be the same as without one? Am I overlooking something else like center to center distance? Any suggestions other than (get dual drive) are welcomed and appreciated.
Theoretically no. The tensioner allows you to get more teeth engaged at a time so the load is spread out. Meaning you can get it looser before you start skipping teeth.
This is why:
Single drive setups stress the ESCs, motors, belts, and wheels more than a 2WD or 3WD setup.
But this is definitely abnormal. I can usually go at least 500km on a 15mm HTD5M belt, if not a whole megameter or more. It sounds like your belts are too tight. If they still skip, they may be misaligned.
Try this:
I’m gonna start using this
I have the same dickyho idlers. One came
With the bearing glued inside the idler pulley. The other sat loose. I just glued it in carefully.
At the moment I have 1853km on both idlers and belts. Still run fine. Belt is more loose than tight. It can be moved sideways on the motor pulley easily.
I also remember the belts becoming silver in the beginning but not anymore.
Using two skate bearings work fine as a well. I 3d printed a piece for the bearings to sit on and then a 4mm bolt through.
Thx, I’m never disappointed asking questions here 
Yeah the belt was definitely too tight. I loosened it since. But it might also be misaligned, will double check that today. I was too focused on getting more teeth engaged.
My way of doing it is press on it from the top, it should only move 2-3mm. By definition, a lil bit.
Yeah, I made an idler with two skate bearings before I bought diskyho’s idler. I have an improved version on my board again now. I’m currently searching for a bearing that is 15mm wide or two that are 8mm, because skate bearings are only 7mm thick. I’m also thinking of adding a big washer so the belt couldn’t slip. My motor pulley does have a lip only on one side and I currently have it towards the motor. Any insight what is best mechanically speaking I see a lot of variances with pulleys and the orientation of the lip.
What did u use to glue the bearing inside the idler pulley?
