Yeah I wouldn't count the tax refund either, otherwise you could just start withholding an extra $500 a paycheck in 2019 and have huge "side income" in 2020.
The only exception would be someone who gets a huge refund from EITC, bringing their total tax paid into the negative.
Right now our side income covers daycare expense and maxing out a 529 for the state tax benefit, with a couple thousand left over (~$20k or so total).