The meridian flip is only configurable with HA>.......
Why not HA< .....

Suppose i want to do a meridian flip half an hour before it crosses the meridian.

That is not possible, as there is no 'flip' command, it is just a 'slew' command and the mount itself decides on which side this is. So if you would try the 'meridian flip' before the meridian it would just stay where it is. This is also why you should use a value larger than zero for the meridian flip limit, to make sure the pointing really is clearly past the meridian...

Clear answer. Thanx.