Text on image - watermark - font size

Question

I have images, that are stored in sql server.

 

One of the images is retrieved, into a memory stream, then returned as an Image object.

 

I then have a method called addWatermark(string text, Image img)

 

Graphics g = Graphics.FromImage(image);

g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;

g.DrawString(text, new Font("Tahoma",fontSize), Brushes.White, new PointF(0, 0));

 

i want the text to take up around 70% of the width of the image... im trying to calculate the value fontSize

 

the image thats passed in could be any size.... not always the same size.

 

Basically, i need to calculate the max font size, then reduce it by 30% i guess.

 

any ideas?