The above mentioned formulas are only valid if the breakout boards come with the specified sense resistors; e.g. for DRV8825 these resistors come with 0.1 Ohms (per coil).
I would put this as step 1. For boards other than the Pololu ones, the sense resistors are likely to be different. Particularly because the datasheet for the DRV8825 shows different sense resistors.
STEP 1 Find the formula for the current sense circuit for your actual driver. Do not use advice from another driver: they might be different. This should be given to you when you bought the driver. It might be in the product description or on a piece of paper in the box. The formula below is based on the very common 0.1 Ohm sense resistors but yours might be different.