Definite integral over discrete data, solving for upper limit

I have two dimensional discrete data set which I integrate to find the total volume under the surface (between the upper and lower limits). Data set has the Gaussian shape and centered around the Zero. QUESTION: I need to find the radius (centered around the zero) of which gives me the 80% of the whole calculated volume under the surface; Basically, I need to solve a definite integral with known answer and unknown 'upper limit'. Any idea how should I do this? Thanks.