We recommend that teams using SI units convert imperial units to SI units, add the tolerance (if they need to take advantage of the tolerance), and then round down to the nearest integer of mm to get the maximum allowed dimension.  For example, for the base plate dimension mentioned in the question, Section 5.7.a of the Official Rules describes a 0.25 inch tolerance.  This would result in maximum base plate dimensions of 18.25 in. x 18.25 in., which is 463.55 mm x 463.55 mm.  Rounding down to the nearest integer of mm would give 463 mm x 463 mm.  This will still be within the allowable dimension and will be easier for teams using SI units to cut the base plate.  Please note that the 0.25 in. tolerance is only for the base plate and other dimensions have different tolerances.  This information can be found in the Structural Model section of the Official Rules.  For example, Section 5.2.a specifies that frame member dimensions have a tolerance of 0.01 inches.  The above recommendation is intended for the convenience of teams that use SI units, but the competition is still judged using imperial units.