I've thought about a light, linear, soft and simple change, after I gave a look at a map I've run (in 1836-1864). Which is as it follows:

I) When Secession breaks, if the seceding faction, or country how wish call it, has too much strenght or people in proportion to the loyalist part of the original country,

then it will be a revolution instead, and not a Secession. A revolution will fire, with same elements of the drawn of secession.

II) If the Secession is too little, oppositely as to what just described before, it also will turn into a revolution instead -

unless the 'too little' Secession, in prportion to the loyalist part of the original country, has got large enough portion of land territory, this very last case Secession can fire.

Both cases, of a Secession made of a too large, or too little, country in front of the original country, the similar revolution Diplomatic play will fire in its place.

That way, you shall ever have Secessions between two comparable size twin countries; or between a country and its minor Seceded country, but with larger land (historical).

Or you will only possibly have a revolution, born out from a Secession boiling, whenever one side happens to be born far stronger than the other one.

Or at least, when Secession has to break out, assign two seemingly portions to contenders.

Which minds, interestingly, also to an historically plausible read of the century.