Values are quoted in terms of weight used for calculations, commonly interpreted as a percent. However, this is more accurately stated to be the probability of appearing in the first tech spot in the shop. The probability of it appearing in the shop is a more complicated calculation. A good rule of thumb is that the probability of a given tech appearing in the shop is the probability of it appearing in the first slot (listed probability) multiplied by the number of tech slots. This is an approximation, but is a pretty good one. The calculation becomes infeasible when there are more possible disks, so the rule of thumb is useful.
Before level 11, the probability of seeing a given tech in the shop for a given visit is:
The probability of seeing a given tech in the shop for a given visit is:
Parsing data and file help done in collaboration with AKDylie.
Data and formula taken from Newserv