This tool will optimise the number of transfers in order to maximize your evolves and XP. Select a species, enter in the number of it you have, and the number of Candies. Leave "Transfer" as no if you don't want to transfer the evolutions as you evolve. Pidgey Spam isn't everything, check out XP Batching in Tips & Misc.
This PidgeyCalc tool loops through the evolution process to find the optimal amount of Pokemon to transfer and reap for Candy before beginning evolve spam.
Pidgey, Weedle and Caterpie require only 12 Candies to evolve. At the same time, these Pokemon are rather common. This leads to the strategy of "evolve batching". Trainers save up evolves to be done enmasse during the effect of double XP.
This tool caters to those trainers by calculating and balancing for them how many Pidgeys they need to have as evolve candidates, and how many they can transfer for Candy.
The tool assumes that evolving a Pokemon takes 21 seconds from start to finish and transferring a Pokemon adds a further 3.1 seconds onto the time required. The additional time cost of transferring Pokemon reduces your XP/min from evolving, we recommend saving up enough Pokemon to evolve non-stop without needing to transfer for more Candy. See Tips & Misc for details.
Before submitting a bug report for this tool, please remember that when a trainer evolves a Pokemon, they immediately get 1 Candy back after the evolution. In addition, if you have set "Transfer" to "Yes" you will also be assumed to be getting 1 more Candy back for transferring the Pidgeotto after evolution. The price of a Pidgey evolution is 12, but the effective price falls to 10 once these effects are factored in.
Full credit to trainers Arieruu and Nylonee for compressing 2 ugly loops into a single line formula.