Bicycle Mechanics - how long for a chain?

Changed my chain and rear cassette and now Im thinking maybe the chain should be shorter? The new cassette is 12/25 the old was 12/27. Is there a rule of thumb I could go by say for instance if the bike is in 3rd gear what position the wheels on the rear derailleur would be in? Figure there has to be a method. thanks......

Changed my chain and rear cassette and now Im thinking maybe the chain should be shorter? The new cassette is 12/25 the old was 12/27. Is there a rule of thumb I could go by say for instance if the bike is in 3rd gear what position the wheels on the rear derailleur would be in? Figure there has to be a method. thanks......

I'd leave well enough alone.

The big issue with chain length is that your chain is long enough to safely cover the big/big combination. If it's not and you inadvertantly shift into that combination, you'll twist your rear derailleur cage into the spokes. That can be a hyper expensive mistake not to mention a long walk home (if you're able).

A less serious potential problem is when your chain is too long. If your rear derailleur doesn't have adequate chain slack take up capability, your chain will go slack in the little/little combination and bounce off of either the front or rear sprocket.

If you had enough slack take up with a 12/27, you'll have enough with a 12/25 too. If your chain was long enough for a 12/27, it'll be long enough for a 12/25 too.

No reason to change it for that cassette difference.

I always size my chain as long as possible. That way I don't have to lengthen it when I swap to a bigger cassette.

big to big + 2 links

Jay...not always. Idealy you want your chain as long as possible without having sag in the samll/small. If you size your chain this way and CANT shift into the big/big combo you need a rear derailleur with a longer cage. Running your chain as long as possible dcreases the amount of stress on it when your in big/big combos.