Help needed with G code

Greetings....

I'm returning to this list after a couple of years absence, having successfully
converted my Taiwanese mill/drill to CNC in the meantime. Being so inclined,
I've been coding manually and producing parts in metal that I certainly could
not make by turning the handles.

Today I ran into a problem, though, and I wonder if someone can help or at least
point me to someone who can. I've had a good trawl of the web and this is one
aspect where all the writers avoid going into any detail.

I needed to cut a curve in the Y-Z plane and, although the G17, G18 and G19
codes are supported by CNCPRO (which I would heartily recommend, by the way) I
cannot work out how to use the I, J and K parameters to get the right result.

The following code should have cut a symmetrical convex curve from Y6.20, Z-1.20
to Y-6.20, Z-1.20 with its centre at Y0, Z-15.60. (The value of X is not
relevant.) By my reckoning, the cut should be anticlockwise viewed from the X
+ve end of the machine table.

G00 Y6.20
G01 Z-1.20
G19
G03 Y-6.20 Z-1.20 I-6.20 J-14.32
G17

It doesn't work, or rather the tool goes all over the place, so what am I doing
wrong?

Thanks in advance,

Mike