Weightage allotment to activities

Here we are following some Common Sence rule foe giving weightage to activity

Consider Boiler:

Boiler-100%

1) Engg-10%.

2) Procurement-50%

3) Construction-40%

Now Consider 1) Engg-10%

                      a) Civil-1%

                      b) Mechanical-7%

                      c) Electrical and C/I-2%

Now Consider 2) Procurement-50%

                      a) Civil-05%

                      b) Mechanical-35%

                      c) Electrical and C/I-10%

Now Consider 3) Construction-40%

                      a) Civil-2%

                      b) Mechanical-30%

                      c) Electrical and C/I-8%

So Please tell me wheather this Concept is right or some other method to allot weightage?