How is the reduction rate of railway engineering calculated?

This is the budget in the hands of the railway construction unit. The cost reduction is negative, usually 3% ~ 8%. This value is based on experience, not calculation.

Second, the noun explanation:

Reduce the rate, that is, reduce the rate of bidding cost, except the rate of "non-competitive expenses".

Third, how to optimize:

Construction scheme: three teams for bridge construction, two teams for culvert construction, and professional teams for beam fabrication and erection. ?

Optimization scheme: Except for the two professional teams of pile foundation and beam making, other overpasses, passages and culverts are divided by mileage sections, which not only fully considers the convenient operation and centralized management of each team, but also avoids the contradiction of centralized use of mechanical equipment and turnover materials.