I find the chakmas to be excellent finishing burnishers. If I'm taking a khukuri out with me to use, I have the chakma in one of the pouches and the severed end (5 inches) of a cheap course kitchen steel in the other. The course steel can, with light pressure, pull the edge back into alignment, or with heavy pressure, pull off metal and reset the edge.
When I'm ready to get it really sharp, I switch to the chakma. I put it flat side down on my knee, and then rest the khuk blade on top of it, tipping the edge until it is perfectly flush against the chakma. Then, I lightly pull the edge along, using only the weight of the khukuri as pressure, two or three times on a side, and then switch, and finish with one more time on each side.
Many of my chakmas are not as hard as the hardest part of the khukuri. This is fine, it may just take a few more passes, but it will still work. In fact--and people can argue with me all they want--I find that the soft ones are excellent for achieving super sharp edges on all of my knives. When you come up to nicks or wavers, there will be a slight pulling sensation, as the blade is actually cutting into the chakma a tad bit, but the chakma is also pulling that little imperfection straight again. I have a couple of Dozier skinners that weren't as sharp from Bob as they are after a little chakma work.
So, have a serious sharpener with you to do actual edge work, and let the chakma just serve as your grand finale. If all you have is the chakma, it will still help, just keep in mind that it will take it a long time to put the edge back into alignment if you've gone too long between honings.
Personally, while I like all my little kardas, they don't go afield with me. Using the Pendentive method, I have several of them full-height convex ground and shaving sharp, but they still don't slice as well as my pocketknife.