I was in the middle of writing this yesterday when my internet connection crashed.
To OP
This is not a bad question since there are many approaches where this calculation doesn't match the published altitudes.
The published altitudes are recommended altitudes to allow a constant rate of descent at the published descent angles. This descent angle will lead you to the threshold at threshold crossing height(sometimes 50ft). The
DME is not always situated right at the touchdown point. If it were, then the published altitude and the calculated altitude would be the same.
Fig.1 : In this figure, you will see that 5.9D from IIDR is not 5.9D from threshold. So, if you calculate using 5.9D, that answer will not be
accurate. The plan view shows how much is 5.9D from the threshold. It is 0.8+3.0+1.9=5.7. So, I have calculated using 5.7, and it comes accurate enough for me(only 2ft deviation).
Now, take a look at the image below. These are the distances from IID DME which is located beyond upwind end of RWY 25. At 5.0D IID, you have to be 2840'. From IIDR this altitude was 3420'. Both these DME distances are not from threshold. Your
calculated error will be greater the farther away the DME is from the threshold.
Fig.2
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@ LLZDME
Can you could post the chart in question?