I'm in a similar situation.
I ballpark what I will make over the entire year consulting, add it to my regular salary and then run it through this calculator.
https://www.simplepay.ca/canada_payr...calculator.php
I see what the difference is between what my employer is deducting and what I will pay in total.
I then get my employer to "top up" what they take off for tax. Or if you prefer you can just set it aside.
Obviously it's not 100% accurate because you likely will have some expenses to claim against your consulting income. Also your actual extra income may vary from your original projection.