About 40 - 50% of your usual tramadol dose. DHC is roughly 2x as potent as regular codeine.
Although the MME for tramadol and codeine is a little all over the place depending on where you look, id say codeine is about 25% more potent than tramadol in its opiate effects but this does seem to vary. Treat them as if theyre equipotent if you havent taken codeine.
So start with half your tramadol dose in DHC, 100 - 150mg