Most of the information I can find on the web regarding re-exposure is really confusing. However i've discovered that as long as you've fully bleached out your silver image, it's almost impossible to over expose on the re-exposure. A 60 W bulb at a distance of 1 meter for 60 Seconds is sufficient.

I hang a lightbulb above my sink, remove the film from the spiral and make sure bothe sides of the film receive light for a total of around 60s. I have had good consistent results with FP4+, Delta 100 and just today with Rollei Blackbird (very clear film base). The fun thing about this process is that it can be done with paper too - here's a direct positive on Forte Bromofort http://www.flickr.com/photos/sebsussmann/6885723103/ but it works well on Foma papers too.