If you use your DR as the assumed position and thereby keep the intercepts short, the longitude thing wont be an issue. Problems arise when you use methods like HO 229 that require plotting from a whole degree of lat/long because the intercepts can be large.
If you do want to use larger intercepts, take the difference in minutes of longitude between assumed position and fix, and divide by the cosine of the latitude. Then shift the fix horizontally by the difference.
So say you plotted from 54 deg N, 2 deg E assumed position and got your fix at 53 deg 48 N and 2 deg 13 E. The difference in minutes is 13, so take 13 / cos (53.8) which gives 22. So your fix is 53 deg 48 N, 2 deg 22 E..
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