If you didn't want to do the lookup table, you could calculate for the equation of the line.

To find the slope of a line, you can enter the two points x1,y1 and x2,y2 on this calculator: http://www.ajdesigner.com/phpline/li...e_equation.php
For the graph shown above, the slope is (m) = 0.011904761904762, which can be simplified to .012
y = m * x + b

We can simplify this to be timing advance (y) = .012 * rpm + .2 or (12 * rpm + 200)/1000 when the rpm is between 400 and 2500
When the rpm is below 400, timing = 5 degrees
When the rpm is above 2500, timing = 30 degrees