It is commendable to try and use two stage ASM missiles and they can work but they are not the best choice, most of the time. They can still be intercepted before releasing its sub-munition if there are any advanced scouts about and once you fire them there is nothing you can do to react to a changing scenario.
You either have a very powerful active sensor at the planet, send out a few fighter scouts to paint the target or as suggested above to have a sensor on the missile itself. The problem with having sensors on the bus and/or missile is that your missile will start to get VERY expensive... both to research and build.
Stationing missile boats or fighters at the planet are probably always going to the best solutions to almost all situations as it is much cheaper and they can react as a situation change and they will get back so you can rearm and attack the target again. It will probably be the cheapest but perhaps the most boring solution to the problem...
Since you are talking about a planet then it obviously can't hide and try to move away from an enemy, as such I rarely bother using long range missiles but tend to focus on attacking anything that get close and defending against enemy missiles first. If I need to protect other colonies or mining operations in the same system I generally rely on either fighters, missile boats or some other smaller ships.
The other problem with the multi-stage ASM is the speed of the first stage, it tends to be very slow, usually much slower than sending in a fighter and firing a much faster missile at more of a medium distance. This means that the enemy might actually strike you before your missiles reach the intended target, if you fire at a way point you also might risk the enemy fleet stopping and reverse course once they detect your missiles which they might do quite early if you are unlucky, or they just reverse course for some complete other reason.
I find that using long range multi-stage missiles work absolutely best AGAINST planets or jump point defences or other points in space with immobile resources or against a stationary enemy that don't know your even exist. So they are certainly useful but in my opinion highly situational.
To talk more about specifics then I would use about half the missile for the bus if you use an active sensor there and half for whatever sub-munition. The release range have to be highly depending on your sensor technology and the size of the missile. If you guide the missile with a sensor from the planet or a scout I would first just see if there is any possibility to just produce a larger slightly slower missile instead of a multi stage one, if that is still not practical then use at least 40% for the first stage perhaps as much as 50% to get as much speed on it as possible, if it is too slow it might be useless at the distances you wan't to fire it at, so much can happen before the final stage is released. It also depend on if you have a good technological advantage against the possible enemy or not.
I would certainly not be producing any such missile unless I know what capabilities and types of ships I'm likely to face.