It may not be possible to create fixed confidence interval that applied to all x values. Width of the confidence interval depends on the value of Xis. The CI band will be minimum around the x bar and the band width will increase as x increase/decrease from xbar.
The link below ans example of simple regression, shows how CI band in different xs.
http://www.vias.org/tmdatanaleng/cc_regress_confidiv.html
If the CI band width is not varying much, then you could try average CI width or take the maximum CI band ( Max ( CI at xmin , CI at xmax) ),apply this to all observation.




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