These things aren't really going to do much for you. The challenge of manuals on an MTB isn't finding the initial balance point. I rode BMX for 20 years before touching an MTB, could manual for a quarter mile down SF hills, through rhythm sections, into rails and drops, etc. Got on a MTB for the first time couldn't manual for 10 feet. Took me a long time to get it down on the big bikes and I'm still not great compared to some. Why it's hard to manual an MTB:
1. BB drop. Your feet functionally go forward for a sec as you lean back, so the balance point is tricky, it's like a parabolic curve instead of a linear progression like a BMX
2. Rear suspension axle path. It's moving as you lean back, again changing the balance point in a hard to predict way. You'll notice as you start manualing longer you have to readjust after manualing for 5-10 feet as the suspension levels out.
3. Tons of rolling resistance. You're constantly slowing down unless going down a decent hill, which is trying to drop your front wheel. Some people lean way back and just ride the brake to learn (like you can do on a moto), but you'll never really fully learn to manual this way
I recommend pumping your shock and rear tire way up, find a 2-3% downhill paved grade that's long, and learning there. It's really hard to loop out an MTB, especially something like a Megatower or similar 29er with longish chainstays and tons of BB drop. Put some flats on and go try to loop out on purpose in the grass/soft ground to find where that point is, you'll realize you should have no fear of looping out one of these big bikes as it's next to impossible.
