例:设计图上给的W10,井底标高225.867m,管内底标高225.867m,管子砂垫层100mm,检查井垫层300mm,现在计算沟槽开挖算槽底的标高,是225.767m?还是225.567m?还是225.467m?如果算检查井的井深,实际井底标高是不是225.467m?