Not exactly. However, the chart linked below will show what the spread will be at varying degree spreads. This chart is for 7" reflectors and grids. To calculate a specific spread for a specific size reflector, use the measurement for 7" reflectors, then add the difference in reflector size to the end spread.
For example, 7" reflector with a 30 degree spread at 10 feet will illuminate a circle of about 72 inches. The 18 OMNI is 18 inches in diameter, or 11 inches more than the 7" reflector. This will result in a circle of about 83 inches in diameter.
Also, remember beam spread is measured from centerline out to -1f. Beyond that, you can have various rates of fall off, from gentle to almost spotlight-like hardness. The beam spread measurements do no describe the fall off pattern.
http://www.paulcbuff.com/popups/grids/7_grid_chart.php
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