The short answer to your question is that there is no safe harbor amount. If you were audited, it would not make any difference if you chose 10% or 90% if you simply picked a number out of thin air.
Do you have any records? Either old tax returns or account statements. Even if you had a few, you could come up with an estimate. For example, if your wife's mother always contributed the maximum amount, you could extrapolate.
NJ may not accept that on audit, but at least you have a foundation for your number if you want to go that way.
The maximum amount allowed as an IRA contribution was $1,500 from 1975 to 1981, $2,000 from 1982 to 2001, $3,000 from 2002 to 2004, $4,000 from 2005 to 2007, $5,000 from 2008 to 2012, $5,500 from 2013 to 2018, and $6,000 from 2019 to 2021. Beginning in 2002, those over 50 years old could make an additional contribution of up to $1,000 called a "catch-up contribution."
The safest position is that if would all be taxable, but that would be incorrect.
You did pay tax on 100% of the distribution in 2019 so if you filed an amended return and it was rejected, there would be no additional tax or penalty. So if you want to amend with some basis, you can see what happens with no consequence. If NJ accepts your 2019 amend, there's no 100% guarantee it will accept the IRA basis going forward, but it's a good sign.
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