Mill: Machining an Arc on a Manual Mill

I don't have a rotary table for the mill (yet) but wanted the ability to make an arc if I needed. The easiest, thought tedious, way is breaking down it into tiny segments with a discrete X and Y movement. If it's broken down small enough, then you can approximate a smooth arc very closely.

To find what the movements were, I created a spreadsheet which computed the ratio of X and Y to the length of the arc at every half of a degree. Therefore, if you know the circumference of the circle that arc you want is a segment of, then you can calculate how far you need to move X and Y for that half degree.

To use the chart I calculate the circumference of the circle (2r*pi) the arc is a segment of and divide it by 720 to get the length of each half degree of circumference. I then imagine a circle on the table, with angle 0* parallel with the Y axis, and find what angle relative to 0* I want to start the arc at. I'll first multiply the X ratio by the length of the half degree and move my X that amount. Then I multiply the Y ratio by the length of the half degree and move my Y that amount. I then step to the next half degree and repeat the process until I've reached the angle I want to finish my arc at.

As you probably noticed the chart doesn't tell you which direction to move, just the amount needed to move.

The spreadsheet I use:
Circle.xlsx

Angle	Ratio X	Ratio Y
0
0.5	87	0
1	76.39	0.01
1.5	45.83	0.02
2	32.73	0.03
2.5	25.45	0.04
3	20.82	0.05
3.5	17.61	0.06
4	15.26	0.07
4.5	13.46	0.07
5	12.03	0.08
5.5	10.88	0.09
6	9.93	0.1
6.5	9.13	0.11
7	8.45	0.12
7.5	7.86	0.13
8	7.35	0.14
8.5	6.9	0.14
9	6.5	0.15
9.5	6.14	0.16
10	5.82	0.17
10.5	5.53	0.18
11	5.27	0.19
11.5	5.03	0.2
12	4.81	0.21
12.5	4.61	0.22
13	4.42	0.23
13.5	4.25	0.24
14	4.09	0.24
14.5	3.94	0.25
15	3.8	0.26
15.5	3.67	0.27
16	3.55	0.28
16.5	3.43	0.29
17	3.32	0.3
17.5	3.22	0.31
18	3.12	0.32
18.5	3.03	0.33
19	2.95	0.34
19.5	2.86	0.35
20	2.79	0.36
20.5	2.71	0.37
21	2.64	0.38
21.5	2.57	0.39
22	2.51	0.4
22.5	2.44	0.41
23	2.38	0.42
23.5	2.33	0.43
24	2.27	0.44
24.5	2.22	0.45
25	2.17	0.46
25.5	2.12	0.47
26	2.07	0.48
26.5	2.03	0.49
27	1.98	0.5
27.5	1.94	0.52
28	1.9	0.53
28.5	1.86	0.54
29	1.82	0.55
29.5	1.79	0.56
30	1.75	0.57
30.5	1.71	0.58
31	1.68	0.59
31.5	1.65	0.61
32	1.62	0.62
32.5	1.58	0.63
33	1.55	0.64
33.5	1.53	0.66
34	1.5	0.67
34.5	1.47	0.68
35	1.44	0.69
35.5	1.41	0.71
36	1.39	0.72
36.5	1.36	0.73
37	1.34	0.75
37.5	1.32	0.76
38	1.29	0.77
38.5	1.27	0.79
39	1.25	0.8
39.5	1.22	0.82
40	1.2	0.83
40.5	1.18	0.85
41	1.16	0.86
41.5	1.14	0.88
42	1.12	0.89
42.5	1.1	0.91
43	1.08	0.92
43.5	1.06	0.94
44	1.04	0.96
44.5	1.03	0.97
45	1.01	0.99
45.5	0.99	1.01
46	0.97	1.03
46.5	0.96	1.04
47	0.94	1.06
47.5	0.92	1.08
48	0.91	1.1
48.5	0.89	1.12
49	0.88	1.14
49.5	0.86	1.16
50	0.85	1.18
50.5	0.83	1.2
51	0.82	1.22
51.5	0.8	1.25
52	0.79	1.27
52.5	0.77	1.29
53	0.76	1.32
53.5	0.75	1.34
54	0.73	1.36
54.5	0.72	1.39
55	0.71	1.41
55.5	0.69	1.44
56	0.68	1.47
56.5	0.67	1.5
57	0.66	1.53
57.5	0.64	1.55
58	0.63	1.58
58.5	0.62	1.62
59	0.61	1.65
59.5	0.59	1.68
60	0.58	1.71
60.5	0.57	1.75
61	0.56	1.79
61.5	0.55	1.82
62	0.54	1.86
62.5	0.53	1.9
63	0.52	1.94
63.5	0.5	1.98
64	0.49	2.03
64.5	0.48	2.07
65	0.47	2.12
65.5	0.46	2.17
66	0.45	2.22
66.5	0.44	2.27
67	0.43	2.33
67.5	0.42	2.38
68	0.41	2.44
68.5	0.4	2.51
69	0.39	2.57
69.5	0.38	2.64
70	0.37	2.71
70.5	0.36	2.79
71	0.35	2.86
71.5	0.34	2.95
72	0.33	3.03
72.5	0.32	3.12
73	0.31	3.22
73.5	0.3	3.32
74	0.29	3.43
74.5	0.28	3.55
75	0.27	3.67
75.5	0.26	3.8
76	0.25	3.94
76.5	0.24	4.09
77	0.24	4.25
77.5	0.23	4.42
78	0.22	4.61
78.5	0.21	4.81
79	0.2	5.03
79.5	0.19	5.27
80	0.18	5.53
80.5	0.17	5.82
81	0.16	6.14
81.5	0.15	6.5
82	0.14	6.9
82.5	0.14	7.35
83	0.13	7.86
83.5	0.12	8.45
84	0.11	9.13
84.5	0.1	9.93
85	0.09	10.88
85.5	0.08	12.03
86	0.07	13.46
86.5	0.07	15.26
87	0.06	17.61
87.5	0.05	20.82
88	0.04	25.45
88.5	0.03	32.73
89	0.02	45.83
89.5	0.01	76.39
90	0	1