I have commented about that on this thread before. The "recommendations" are meaningless, IGNORE them like they don't exist. Even the "default" settings are misleading. bOS has no way of knowing your board's best efficiency, it doesn't have any special code for calibration or an AI guessing proper settings, it is as manual as it can get.

If you don't understand this, I'm not surprised that you burned boards already. This firmware is the opposite of hand holding, it gives you full control and no protections. You CAN easily burn your hardware with it, faster than you anticipate... It is a powerful tool, but with it comes responsibility. There are even some bugs that they are addressing (like the reordering thing of the hash boards which they will remove); so "proceed at your own risk". This is a firmware for tinkerers not newbies expecting a quick replace and let it decide whatever. It is not that kind of firmware. It is certainly not tailored for overclockers, it won't protect you at all or "self guess", and the values you see are probably a result of some simple calculation that has no relation with the actual hardware involved.

You have to lower that voltage until the board stops hashing or the error rate is too high, then step a notch back and leave it there. If the board seems wants more than 9v, its time to reduce the speed, period. Every single hashboard is different, even from the same batch, even in a single unit, in addition to knowing that the middle board has poorer ventilation which you should use to your advantage by favoring the best performing board in that position and putting the less fortunate to the sides. OR do the mod to have the intake fan be farther away.