NavList:

It appears that your problem is that you used different assumed positions for tach computation. Look at your input data to the USNO website, it is different than the AP shown in block 6 of the 249 reduction. When you do that you find the Hc is 21° 22.6'

The good news is that you have mastered HO 249. When comparing you measured altitudes .to computed values to see how accurately you have measured the altitude it it best that you use your known location as the assumed position, as you did on the USNO website, so you can then compare that directly with your measurement  which shows that your's was 17', so your LOP will be only 17 NM from your location which is good work with an astrolabe. There are ways to adjust the HO 249 computation so that you can use your actual position as the AP and I have uploaded the tables for this to Navllist in the past. but it is easier to use the USNO site. When using an astrolabe or bubble sextant or artificial horizon you do not apply dip or semi-diameter so the only corrections that apply is refraction and parallax in altitude. Looking at your result from the USNO you see that when you combine these two corredcions they come to - 2.4'. If you apply them then your Ho becomes 21° 27.6' and the intercept increases to 19.4'.So when shooting altitudes for practice you should continue to use your actual position when using the USNO website. If testing your use of HO 249 then use the same AP for the USNO website.

gl