It all depends on which DEC, correction, Hd of Cap you are dealing with. Most of them need a tame "chap" to tie their shoe laces. I am talking about the majority of these good 1*s infecting MB not just JSF.
When they come to you and say "oh no we want capabilty X delivered for y spondoolicks and how dare you ask for any more, don't you know we have had some consultant work the figures up and they are robust and accurate because it cost us £Zs so it must be right!" (what a crock!)
So when you have looked at what they have dictated needs to be fitted, beg pardon. Removed all fat, then all useful capability to fit the cost. Checked the costs with the real world, then twisted the manufacturers arm and those costs come out at a fairly robust level of certainty. Then you factor in risk and uncertainty and they say oh you oik why is it £xxxx over what our expensive consultant dictated it would be well yes I am rather put out!
I think some clear expectation management is needed. To get a lot of these capabilities in requires a large amount of investment to reduce the technical risks, hearing that some guru can just create a project cost for a certain cost just makes me want to ring up the Daily Wail!
Plus the only reason the ISD is so far out is because of some Treasury d**k dance where Darlings boys and girls have said you can only have this much money for your new toy.... grrrr that is my frustration. Then they have the audacity to say well you are just being difficult because you are too close to the project........feckers!