in our case we use xtralength for additional length that has no items or element but part in calculating the bolt length (example extra thread length) and the bitlength are for the length on each item on the Bitems example;
BITEMS - NUT WASH WASH NUT
BITLENGTH - 15 2 2 15
with these example, bolt calculation will be ( Flange Thk x 2 ) + Gasket Thk + 15(Nut) + 2(Wash) + 2(Wash) + 15(Nut) + Xtralength
