Teroc,
You can guarantee that one day someone will want you to do it the other way round ie intercept a radial from an arc. Here's how: The 1 in 60 rule is the basis for this concept. The number of degrees to lead the turn by equals (60/DME of the arc) x 0.5% of the TAS. This assumes a rate one turn. eg with a 200 kt TAS flying round the 15 mile arc: 60/15 x 0.5% of 200 = 4 degrees.
For ease of MDR just remember that:
90kts requires anticipating by 27 deg/radius.
100kts requires anticipating by 30deg/radius.
120kts requires anticipating by 36deg/radius.
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