Here's the deal. Enlisted personnel receive all of their pay and bonuses tax freee when in a combat zone.
Officers are different. O's may receive any pay and bonuses in a given month tax free, BUT only up to the maximum base pay that the Chief Master Sgt of the AF could earn in a month. (Technically plus the hostile fire pay, $225) Check the base pay scales for the top right corner of the E scales for the current max. Then, whatever your base pay is less than that is how much of your bonus you will get tax free. The rest of the bonus is taxable.
If you're a Major or Lt Col, you're probably already earning about what the CMSAF makes, so you get little to nothing of your bonus tax free. And, if you're making more than the CMSAF monthly, then you're not getting a full tax-free month when you go downrange, only up to what he would earn, with the left over amount taxed.
During Vietnam the rule was that O's only got $500 of their income tax-free. That was based on how much E's earned back then. The rule wasn't changed until after Gulf War I to reflect current pay scales, and it was set up to change as pay scales change rather than as a fixed dollar figure. Thus the "maximum that the CMSAF earns" rather than saying $4,523.36 or whatever it is.