Thunderhead,
We often use straight beam to UT welds, specifically from the back side of t-joints or inclined t-joints. We use API RP-2X (very much similar to AWS D1.1 Appendix S), and as such we use ASME style basic calibration blocks for sensitivity calibration. For straight beam calibration, we use modified blocks that take into account the fact that you may be travelling through a significant amount of through-member base material before you get to the weld. I recommend that you evaluate the option of using a basic calibration block instead of distance-amplitude blocks - they are easier to make in house for most operations (you don't need to make flat bottomed holes).
Mankenberg