Both figures should be entered as there will be a difference due to fuel burn during passage and possibly seawater density between ports.

You say it is a chemical tanker and figures was obtained by draft survey? Actually, chemical tankers figures are obtained by proper cargo calculation using tank volume figures, cargo density and actual temperature with application of correction factors. Given the final figure have an apparent discrepancy of -20 at LP and -18 at DP the best approach for the Master is to present a letter of protest (discrepancy protest) for both ports. The BL figure will stand as it is be it at load port or discharge port. But the ship figure should be protected by means of a standard LOP. So it means that the Master should protect (or take) the ship figure at actual ports rather than the BL figure. Master has no influence on the BL figure unless the discrepancy is to big (exceeding the charter party and commercial operator criteria) where the VEF can be applied.