Alan & Old school, you CAN use the dial indicator and you can release the half nuts when cutting metric but you must use this process which is particularly useful if you are working up to a shoulder or into an undercut.
After setting up your lathe to cut the required metric pitch and with your work piece in the chuck or collet follow this sequence.
1. apply the first cut depth on the cross slide and place the tool tip reasonably close to the work [distance depends on the pitch of the thread you are cutting, fine thread can be closer, coarse thread leave more room]
2. with the lathe running engage the half nuts on a particular number, number one is best, it can be any number but you must remember it.
3. At the end of the thread release the half nuts, stop the machine and wind out the cross slide to clear the thread
4. Start the machine in REVERSE and engage the half nuts on number 1 traverse back to the start point making sure the dial indicator passes 1 and then release the half nuts, stop machine, apply the second cut on the cross slide, start machine forward and engage half nuts on number 1
Repeat until you achieve full thread depth.
You MUST NOT manually move the carriage during the process
I appreciate that if there is no shoulder this may seem a faff but I have cut a great many metric threads this way and find it much better when there is a shoulder or undercut than trying to stop the machine before the tool runs into the shoulder or trying to whip the tool out before it does collide and damage both the tool and the work.
John