You could also just reduce the heat applied to the kettle to reduce the boil-off rate. Measuring boil-off rate is easy if one knows the dimensions of a kettle. All one needs is a stainless steel or non-toxic plastic ruler. The volume of a cylinder in cubic inches = 3.14 * radius_in_inches2 * height_in_inches. There are 231 cubic inches in a U.S. gallon; hence, the volume of wort in a kettle in gallons = 3.14 * kettle_inside_radius_in_inches2 * distance_in_inches_from_the_bottom_of_the_kettle_to_top_of_the_wort / 231. The boil-off rate can be controlled by taking periodic readings and extrapolating the lost in wort volume over the boil length. If the extrapolated loss is too great, reduce the heat and recheck in 15 minutes. If the loss is too low, increase the heat and recheck in 15 minutes.
One last thing, the height of one gallon in a kettle in inches = 231 / (3.14 * radius_in_inches2).