Take a look at Seiichi Yoshida's 17P page.
He is usually very fast in giving such values:
http://www.aerith.net/index.html
http://www.aerith.net/comet/catalog/0017P/2007.html

He uses:
m1 = -37.5 + 5 log d + 100 log r [172.5, ] (2007 Oct. 24 - )
that will give:
http://www.aerith.net/comet/catalog/0017P/2007-mag1.gif

Clear skies
Wolfgang

--
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO

----- Original Message -----
From: "alensondivo" <alensondivo@...>
To: <guide-user@yahoogroups.com>
Sent: Sunday, November 04, 2007 9:33 PM
Subject: [guide-user] P17 / Holmes


> Hello,
> Does anyone know the correct slope parameter (G) and absolute
> magnitude (H) for this comet. I keep on downloading the comet
> data but it remains at 6 and 10 which results in mag 17.0!
> Kind regards,
> Thomas