Back-of-an-envelope

On one defense item, somebody kept referring to it as a back-of-an-envelope cost estimate. Do you have a back-on-an-envelope cost estimate?

The reader will not need to be warned that the calculations involved are of the back of an envelope type and statements of the form ‘a equals b’ should be read ‘a equals b approximately’ or even ‘with a bit of luck, a and b will be of the same order of magnitude’.

Misapplying a basic feature of diffusion theory, [Werner] Heisenberg arrived at an impossibly high figure for a critical mass of pure U235. This was done by means of a seductively simple "back of an envelope" calculation in 1940, […]

Now, those Treasury mandarins unfamiliar with the railway might assume on the back of an envelope that if only 80% of trains are running, then that will cost 20% less. If only it were that simple.