Height: 19912/76 = 262 lines
Actual Frequency = (14.31818/12) / 19912 = 59.9227 

front_porch + sync_time + back_porch + active_area = total

The sync signals output from the CGA card are 64 pixels (hdots) wide and 3 scanlines high, so I'm guessing that's what you should put for horizontal and vertical sync time respectively. I'd expect a capture device to handle anything in a certain range, but perhaps the dialog sets the center point of this range. HSync and VSync polarity from the CGA card are both positive.

The front and back porches will (presumably) just set the cropping. If you wanted to crop to exactly 640x200 pixels, the values to use would be:
Horizontal: front porch = 96 pixels, back porch = 112 pixels.
Vertical: front porch = 25 scanlines, back porch = 34 scanlines.

However, you probably don't want to do this since this you probably want to capture some of the overscan area. At the very least, in +HRES mode you'll end up cutting off 8 pixels half the time if you try this.

In standard +HRES mode, the horizontal sync pulse is either 48 pixels (phase 0) or 56 pixels (phase 1), and the porches are:
Phase 0: front porch = 104 pixels, back porch = 120 pixels.
Phase 1: front porch = 96 pixels, back porch = 120 pixels. 

capture a 768 pixel by 240 scanline region with the active area roughly centered:

Horizontal pixels: 768
Vertical lines: 240
Actual frequency: 59.9228
Horizontal Timings (pixels):
 Front porch: 32
 Sync time: 64
 Back porch: 48
 Active area: 768
 Total pixels: 912
 HSync polarity: Positive
Vertical Timings (lines):
 Front porch: 5
 Sync time: 3
 Back porch: 14
 Active area: 240
 Total lines: 262
 VSync polarity: Positive 

Thanks to reenigne for fine-tuning my initial settings.