You can drop out at any distance less than 1Mm and any speed less than 1Mm/s. For tense dropping out, travelling close to 1Mm/s gives you exactly 1 second, assuming you're perfectly aligned, to hit A-Up and get into real space, or you REALLY overshoot!
The tried'n'tested technique is to slow down to around 75% throttle when the countdown timer says around 0:07 (set your throttle increments to 25% to try it, but you can do it in smooth increments with practice) You can go to 75% throttle at any time, or do the whole SC at 75% if you want. For short (say <500Ls) distances, the difference isn't worth worrying about. When you're at 0:07/75% throttle, the 0:07 should stay the same, i.e. you're slowing to half the speed as you cover half the distance. Easy to keep an eye on.
The trick is to be ready to "A-Up" the very instant that you see "Safe Disengage Ready". Even at 300km/s, you only have 3 seconds before you overshoot.