For what its worth, (and this is from my poor memory); One of the people involved in 489's 1420 several years ago told me they put a 1 inch spacer between the back truck bolster and the frame. Not sure how true the 1 inch is, or if it was an assumption. This was the same rebuild that saw 489's tender get a new upper half, though I don't remember the year.
Another thing to think about, it would be self helping or self correcting on most tracks. In theory, when the water supply was full, the tender may sit level or with a slight slope. As the water level went down, the tender would slope slightly forward, pushing more water forward. This in turn would place more weight on the front truck, and less on the back, causing a larger slope. So it is possible that "all" the D&RGW tenders (for example) could have the same spacer size. But the tenders would vary in slope angle due to how much coal and water was in the tender....
Casey