I don't recommend the use of an enlarger for paper flashing. After all, it is wet! Leave it in the tray and just use a flashlight, a safelight, or room light.

Flashing early gives poor results, resulting in images that lose the effect by allowing development to overtake the pos and neg images. Flash too short and the image is weak. Flash too long or with too much light gives a dense pos image.

Do it just right! And that can only be done by experimentation.

Attached are 2 color prints which show the original and the one with the Sabbattier effect. The original was a cross processed negative from EPP exposed at ISO 100 and processed in C41 and then the second print was flashed 2/3 of the way through development with an open safelight at about 5'. The flash was about 3". The process was romm temp tray using RA4 and Endura paper.