[Leica] printing questions

[Guy Bennett <guybnt@idt.net>]

luggers,

reading tim rudman's 'the photographer's master printing course,'  i've
come up with a couple of questions and was wondering if any of you b&w
printer could help me out.

the first concerns conceiving of printing time increments as f-stops rather
than numbers of seconds, the idea being that - if i understand him
correctly - should you later wish to reprint in a different size, you don't
have to recalculate your various burning in/dodging times, you just use new
'f-stop' increments based on your initial printing time. he mentions that
there are f-stop timers available, but that they are very expensive, and so
he includes tables with initial print times and increases and decreases of
time in f-stops for those of us who do not have an f-stop timer. my
question is: do any of you use a timer that works in f-stops? how do you
like it? do any of you use this system with a 'normal' timer? is it worth
the bother?

the next question is related, and it concerns the extrapolation of new
printing times when making another, different-size print of a given
negative. he offers the following equation for making the calculation: t2 =
t1 x (w2/w1)2 [i.e. squared], where t2 = new initial exposure, w2 = new
print width, t1 = initial time of the original print, w1 = width of the
original print. my question: do any of you use this, or any other system to
extrapolate new print times for enlargements? is such a system reliable,
given the variables involved (different batch of paper/new chemicals/etc.)?

thanks in advance for your comments and suggestions.

guy