My first attempt routing a 75mm circle to 5 mm deep, the circle when completed was only 72mm.
do you need to add your bit size onto the size of circle to be cut I used a 1-8th end mill bit
Na, it’s not needed. you do have options to cut inside the vector, or on the line and outside the vectors.
Also, if you are trying to be as precise as possible and your machine has been calibrated to the best it can be, you should measure your bit with a pair of calipers and enter that value as the bit diameter instead of using the generic 1/8". Endmills are created with nominal dimensions and are rarely exactly as published since the manufacturer typically creates them to nominal ± a specified tolerance. For instance, my expensive 1/4" two-flute endmill is not 0.25" in diameter, it is 0.245".