Eg your last 5 projects overran by an average 30% so for comparison we will wack 30% onto your bid.
This will only make everyone who has blown the estimates in the past bid 30% under cost to start with.
Not that this would be a bad thing of course... If there was a way to make them deliver instead of renegotiate. A few of those and voila they are no longer in business so they cannot purchase politicians to drive through IR35 and other legislation so they can maintain a near-monopoly. OK, that is not purchase. We call that donation. It is all above board and and totally legit. Exactly as their directors sponsored Tony's Nu Labor for 8+ years.