Thirty feet at best

Excerpted from one of the Google accountings of baseball stuff -

May 29, 1884: Taking advantage of a ground rule change which scores balls hit over Chicago's inviting 180' LF fence as HRs (instead of doubles)‚ 5 players hit round trippers in the White Stockings' NL home opener against Detroit‚ winning 29-5. Chicago will hit 142 HRs-last year they hit 13-during the 112-game season (more than 90 percent of them at home) to set a record that will last until the 1927 New York Yankees. The rule change appears to be unilaterally made by Cap Anson‚ as noted by historian Bob Schaeffer‚ and the other league owner will squawk to no avail. But the league will set a minimum distance of 210 feet for an outfield fence after the 1884 season.

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The park dimensions were 180' at its closest point (in left). The "new" rule requires the closest point to be 30' further back.

There was a 129 HR difference between the previous year (1883 = 13 team HRs) and the current year (1884 = 142 HRs). It doesn't seem to me to be very likely that most of those 129 HRs fell into that 30' imaginary space. Certainly some did.

I would be so bold to accept that one third did. But I really think that it was less than a quarter.
I think that they mainly just hit more home runs because they tried.
Maybe convincing themselves that they could do to the fence's proximity.
But I do not believe that the park's physical dimensions made as substantial physical difference as is portrayed in most accounts of the 1884 Chicago White Stockings.

What do you think?

I think that if you look at this drawing of the park, it is clear that pulling the ball down either foul line has significant potential for hitting the ball over the wall. However, this is one of three parks in (then current) use which have similar ground rules applied to them: Chicago's Lake Front Park‚ Buffalo's Riverside Park‚ and Cleveland's Kennard Street Park all have ground rules call for only two bases on hits over certain portions of the outfield fences.

Similarly, but only slightly less apparent is the proximity of the park to the lake. If you have spent any time in Chicago, particularly in the winter, Im sure that you are aware of the wind coming across the lake, which would blow a ball to left field. This combination of a short fence and a strong wind will result in unusual home run production, particularly, as you say, because the players are aware of it, and are taking advantage of their situation. And I further agree that it will make no measurable difference if the fence is moved thirty feet back.

The park was temporary accomodations: "
A Chicago court rules that‚ although the NL lakefront ballpark illegally blocks the lake view and breezes from homes to its west‚ the White Stockings may continue to use it through the end of the season". I therefore conclude that there is no reason to favor invalidation of the records produced any more than those of the Colorado Rockies. You take the good hops with the bad.

A look at later Chicago parks....

Looks like the Cubs left field line is well under 200'.
Or is that an illusion?