LWG3780 format's width estimation is too approximate and not forward compatible

[jwakely]

Could SG16 please look at https://cplusplus.github.io/LWG/issue3780

This probably needs a paper, and then a trip to LEWG.

[tahonermann]

Yes, I've got this on SG16's todo list.

[cor3ntin]

Paper was written #1341