How do you mean "doesn't stay still"? You are still moving of course, can't fully stop in supercruise after all and it does sound like you may be getting too close, but that should just make the target bigger ever so slowly as you coast in at 30km/s. Are you just coming in way too close to the planet?
The way I do it is to throttle to 75% at the usual 7 seconds, and then once I'm within 0.9-1ls throttle to zero, which is always way before the "out of range" message has cleared. This gives you enough time to decelerate that by the time you have, you've shed enough speed so you also don't get the "too fast" message and everything should turn blue. By the time you come to a complete stop, the larger circle on your reticule should just be overlapping with the planets edge. If it's not, then you're just still out of range, so throttle back up to 25% for a second or 2 and then immediately back down to zero. I find this puts me at the most extreme upper limit of the range and I never get any closer. Never had any problems mapping like this.