Easy peasy !!!
First you need to find how much the floor differs from highest to lowest point at the perimiter of the floor - the basic principle of this is to run a straight line around the room ( this is where a laser level comes in very handy )
1.Starting in one corner of the room, use a staff and mark on the staff the point where it meets the line.
2. Move the staff around 2 feet ( to the left, or right ) and mark where it meets the line again.
3. Repeat this process untill you have gone all around the room.
4. You will end up with a series of pencil marks on the staff.
5. The measurement between the highest and lowest of the marks is the difference between the highest and lowest points of the floor at the perimiter.
Note.. The highest mark on the staff will indicate where the staff crosses the line at the lowest point of the floor. The lowest mark on the staff will indicate where staff crosses the line at the highest point of the floor.
This will give you a good indication of how much self leveller you will need.
You can go one step further.....
Once you have ascertained the highest point of the floor, then proceed to mark the staff again at this point. Then almost repeat the previous procedure, but this time position the staff around the room with this mark next to the line. Then mark the wall at the bottom of the staff. You will end up with a series of marks at floor level indicating how high the self leveller needs to go.
Hope this makes sense.
Also Mapei do a self levelling compound that levells from 3mm to 30mm in one application - flexible and easy to use providing you add the correct amount of water - It is called Ultrapan Maxi - just add water..
Try this link for PDF Data Sheet
http://www.armatile.com/docs/doc45488adc66175_Mapei%20Ultraplan%20MAXI%20datash eet.pdf 
SLC how deep can you go 
Linear Mode
