Would it be possible to add an option to the menu to define the bonus gained per adjacent reactor?

Yes that would be possible. Why would you like that as a feature?

The first reason is that I liked the 100% bonus for each nuclear reactor. The second reason is that when calculating the ratio between heat exchanger and 2xn reactor, the ratio is 1 reactor to 12.5 heat exchangers using the 50% bonus from the mod, while with the old 100% bonus the ratio is 20 heat exchangers to 1 nuclear reactor. Therefore, I wanted to change the bonus both to gain more energy and to keep the ratio with round numbers.

100% bonus does not preserve the mostly-vanilla quantities of buildings in a 2x2 reactor, which is a good benchmark. At 100%, your 2x2 reactor would produce 720MW of power (180x4) which is significantly more than the typical 480MW.

What I'd like you to consider is that *any* reactor setup will use a whole number of exchangers to consume all the heat generated by the reactors.

1x1 = 5
1x2 = 15
1x3 = 25
2x2 = 40

Even strange setups, such as any 4-reactor setup with one reactor shifted out of the normal 2x2 formation (such as a tetris "T" shape or squiggly) will have a whole number of heat exchangers: 35.

I'm not sure about strange formats, but the 100% bonus still maintains a whole number of heat exchangers — see the table (https://imgur.com/a/Dka0Z2k).
Regarding production, using the 100% bonus results in an average increase of 50 to 60% in total output.
Finally, since this was a personal preference, I had suggested making it an optional setting in the mod menu rather than a permanent change to the mod.

I don't plan on adding this mod setting.

I suggest that you approach the "extensible nuclear reactor problem" from a larger step size: one reactor is 12.5 exchangers, but TWO reactors is 25. That doesn't divide nicely by two for an even left-right or top-bottom arrangement, but if you increase the step size once more to FOUR reactors (50 exchangers) then you can place 25 exchangers to either side of the reactor column which will consume the generated heat at perfect ratio. If you go even further to 8 reactors per extension, you can probably have a more square and tidy design on either side for the exchangers and turbines.