50% of a circle is 50 over 100
50 percent of diameter of a circle is the radius.
Half (1/2) equals 50%.
Don't know if there is formula for converting degrees to percent, but here's a thought: Imagine a clockface with just the minute hand. When the hand makes a full circle, it has traveled 360 degrees for 100% of the circle. When the hand points to 3, it has traveled 45 degrees for 25% of the circle. The 6 is at 180 degrees and 50%. The 9 is at 270 degrees and 75%. You could break it down from there.
The question is missing some information. Assuming this is about making a pie chart, multiply the percent by 3.6 to get degrees of a full circle. So 100% is the entire pie, 50% is half (180 degrees), etc.
If the circumference is split into 4 arcs then the 4th arc is 360-20-140-50 = 150 degrees
if you circle is divided into eight segments, 4/8 converts to 1/2, that is 50% of the circle