🚀 go-pugleaf

I have what may be a heretical question for this group:

What is the 'best' way to make gnuchess play a weaker game?

I am currently learning to play chess (yeah, old dogs and new tricks) and I 
like the gnuchess/xboard combination.  The rec.games.chess.computer FAQ
suggests setting your software so that you win about 25% of the time when
learning.  I am a _long_ way from beating gnuchess 25% of the time with the
default settings.  I know that gnuchess' game weakens with less time and/or
less search depth, but is there a way to apply this in well-behaved and 
predictable manner (that is what I mean by 'best')?  I mean, giving gnuchess 20
sec per move vs 10 sec per move is not likely to make it play twice as strong
(if I understand how these things work), nor is limiting its depth to 10 plys
vs 5.  Any other parameters to adjust?  A table of parameter settings vs
relative strength would do the trick.

(I've read the FAQ and the docs with gnuchess and looked around the Web a bit
and didn't see anything about this.)

Thanks for any help,

Joel