I seem to have two conflicting sources (or maybe its me and my brain is fried from all of this):

In one vid tutorial a guy used the net profit figure from schedule C (carried over to SE), subtracted the one-half (7.65%) of self-employment tax and used that resulting figure to base the 20% contribution from.

So for example, $100,000 (net profit) minus the one-half of self-employment tax ($7,065) = 92,935 x 20% = $18,587 (maximum amount that can be contributed)

But then I read on other websites that the 20% contribution would be from “net income or net earnings”, which is your take home (after all deductions and taxes are subtracted) and from my calculations this figure is way lower than $100,000 per the example (i.e. $40,000 vs $100,000)

Help! I don’t want to significantly over pay or underpay.

Also I’m wondering if I’d be better off opening a solo 401k if the 20% contribution max is based off of the lower net income figure.