Do I guess at my common-law partner's 2017 net income if he is going thru an accountant? He has his last pay stub which shows his net income, do we just use this number?

When you don’t know your spouse’s income and can’t find out through reasonable means, the Canada Revenue Agency recommends that you enter your best estimate.  As long as you enter an amount greater than $11635, there shouldn't be any spousal credit allocated to you.