[Qgis-user] calculating percentages

[Rutger Emmelkamp]

Hello hello everybody, 

Does anyone have some experience with entering percentage data and calculating with the attributes table?

For a peat survey I have recorded the presence of several plant species expressed in cover %. (For example 20% grass / 30% moss / 50% heather)
I have done this for multiple coordinates in the survey area. Each waypoint has a number of values with a total of 100%. 

How do I calculate the 30% cover of a number of points together? And does it have to do with the type of data fields in the attribute list? When making a new field one can choose several Type options: Text (string), number(integer), number(integer 64bit), Decimal, Date and Time. 
Is this the key? And if so, which one to choose?

Many thanks,

Rutger