,(can manage basic preamp/amp circuits),but
i suspect the some difficulties lie in knowing exactly where to put the input and output.
http://www.geofex.com/fxprojex.htm
and similar are probably a better sort of place to start
ie, circuits that won't damage anything ,are safe,low voltage and give you some theory.
Volume pedals are a nice place to if you're new to things :)
V Simple
Accuracy,you say? hmm interesting concept..