Hey there. What year is the bike? I have a '97 and an '01. The Tranny's have very different ratios between the two. The OE final drive ratios were as follows.
1997=13/49
2001=13/48
I chose to run 13/50 on the 1997. That gearing gives me the best all around power delivery and still allows me to run a long logging road in fifth without the bike screaming. I don't have any issues with low end torque in a motocross situation either.
However, on the '01 with a 13/50 combo, the bike wants to pull my arms off. But on a long straight, even in fifth I run out of Revs. Mind you I love it in tight twisty stuff, or when jumps are close to the turn exits (Indoor Tracks). For Trails with long straights or to stretch out my fuel, I run a 14/50. But forget about going slow up big rocky hills. I loose a bit of throttle control managing the power band when slow and technical. But top end in the straights the KX has lots on tap. IMHO, I think the stock 13/48 gearing would be the best all around ratio for this bike. I've just invested much for the 50 tooth Twin Ring Renthal, so I choose to run it, lol.
Thats my $.02. Hope I shared enough to help out.
Mike