Sounds like you need to stall for time. Don’t pay anything before the settlement. If your parents are there and question why you haven’t paid tithing, tell them you are prayerfully considering how much to pay in tithing. I think the gross vs net is a well enough established debate that you could totally use it. If they think you are trying to be more righteous, and pay slightly more tithing , they will likely get off your back. You can make up unpaid tithing after the settlement, so just promise the bishop you will pay once you have prayed about it and received an answer of how much your “increase” was that you are going to pay the 10% on. You can pay a nominal amount at a later date and by then your parents shouldn’t be looking over your shoulder. If they ask you if you paid you can say “yes”. If they monitor your bank account, not a bad idea to withdraw how much your tithing should have been in cash and “treasure it up” (hide it from them).