How to do the zoom in detail (enlarge on portion of the area to certain scale)?

Hi Minhdang, I was looking for the same like you (I suppose :-)). I found something like this: I select the element and fit the view to select element and change the annotation scale in the same time. I created some icons for it and used different combinations for the settings: 
set displayset on;selview 1;displayset set selection;fit all;point ab;displayset clear;update view extended;model set annotationscale 150

I attach the base files as a reference. I can change the scale of that reference to something else (in your case 1" = 40') and clip the reference to show just the area I want to see. Then I can move that "zoomed in" reference attachment anywhere on the sheet I like.

what version msnt?

are you using sheet models?

Are you using annotation scale in your model settings?

do you just  want a view in the model to be that scale or a finished drawing detail either in model or sheet?

Do you know how to make a saved view by 2 points,  then you can ref that save view just like a view port in acad on the sheet or in the model itself..

we need a lot more info to help you exactly.

Not sure what the work flow is here, but for this I would reference the gas line exhibit, then scale by 200÷40.