where would i find data on how adding an extension to a fly rod might have an impact on the line rating of a fly rod? would lengthening increase the rods line wt?

Depends on how much you intend to extend. A couple or three inches isn't going to change much of anything. Too far beyond that and you might see some difference - enough that it might handle another line weight. But before I'd extend by that much, I'd go back and buy a blank that will handle what I want to do.

The Common Cents System would show you the exact amount of change in both power and action, between what you have now and what you'd have for any given amount of lengthening.

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its a 7 foot fly rod (three wt) extended to 7'9". we have talked about this and i have your article. i just thought there was maybe some rule of thumb that i could use. i am assuming it will have an impact since the defection of 1/3 the rod length will change some. maybe not significant. i doubt if it will
be a whole line weight. somewhere i thought i read that some theoretical calculations had been made on % changes in length that i could refer to.
i do understand that it will make the rod sightly "faster" action. my extension junction is well behind the rods natural flex point. its no big deal. it is going to be what it is going to be and i doublt it will be a whole line weight.

Extending it will make the action faster, not less so. 9 inches will also affect the power by enough to notice.

The CCS would absolutely put you on the numbers you'd need to compare and see what you have now versus what you'll have with a 9 inch extension.

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I thought you had this all figured out.
Still asking questions I see about what this will do to the rod geometry ?
It will add a line weight I agree maybe more.
Why not do a tape and glue up prototype and sample that to see the results.
Why not purchase a 8' 4wt blank and cut it down then you may get the 7'9" 3 wt you seek ?
Just some suggestions.
Thats a wrap.