Only above the wind layer will changes in heading change your impact point if you turn. If you are below the wind layer then they will not. Above the wind layer though (and even only slightly above) you need to make sure you are "roughly" on the correct heading when you calibrate as when you will actually release your bombs. There have been some charts/formulas posted in this forum that can help you manually account for differences though so you could search for those also.
You mis-understand the issue with 500ft of difference. The point is, there are two parts to any calibration, ground track speed and altitude above target. 500ft of altitiude change in a bomber impacts your speed by 1-2mph which is not enough to throw off your calibration of that aspect too badly. Adjusting the ground-track part of your calibration is the tough part, that's where you need to track a ground object for ~20 seconds. The altitude calibration component is easy to adjust ,you just re-select your target again and it will automatically calculate the difference from your current altitude to the target (takes a half second to do). So, the point is, say you end up diving 500ft and have enough time to let your speed settle out (lose any bonus diving speed) but don't think you have enough time to totally recalibrate... you don't need to. Just reselect your target in calibration mode, your speed will be close enough that just that effort (which takes 1 sec) is enough to get your accurate enough to hit things as long as your original ground-track calibration was accurate.
One other point, calibrate with your bomb-bay doors closed. You don't need to open them until the instant before you drop. Having the doors open causes ~10mph in drag which is enough to make your bombs miss. Thing is, it takes 20 seconds or so for the extra drag to manifest itself so snap the doors open 1 second before you drop, then snap them closed again. You don't want to be flying around with your bombbay doors open and you don't want to have to open them way before your target in order to let your speed settle out.