Yeah I think we use quite a bit of our brain power de facto. Meditation is utilising the part of it that actually directs the power and has an awareness and thus control over it. Like a transformer switch or using an application on a computer system to perform various tasks which could be stuff in the physical as well as on the computer. You could experience 100% or pretty close if you can fully control that max of energy, but that is god-level because you'd think if an advanced guru plugs their ears into a pylon they're still gonna fry their pink goo into even pinker and more gelatinous goo, XD.
I think you judge someones max spiritual 'wattage' by the energy of their lower chakras maybe. Someone with a higher natural or built up 'wattage' clearly shouldn't ever aspire to do that in their brain.
Seeing astral visions and all that hasn't got anything to do with melting brain type energies, but the more calmer ones dunnit?
I believe HPS Maxine has stated that it is true that most people use only a small percentage of their brains, despite Jewish scientists disputing this, you should be able to find this in one of her posts/