Profit sharing? Hmm... Ok, how much profit should the owners distribute to the workers? 10%? 20%? This was the problem in sports.
The players unions wanted 53% of the profits because they play the game. The owners wanted 53% of the profits because they own the business. Next thing you know, there are no games, the owners are taking the writeoff and the players are stuck living on their million dollar a year plus salaries.
Ok, how much profit should the owners distribute to the workers?
This isn't a mandate on the owners, it's a tax deduction that is over and above the actual amount distributed -- if the owner wants to use it. So the answer to your question is "whatever the owner wants".
Also, the profit is the result of discounting all expenses running the business, including taxes and benefits. So the amount available to satisfy a profit-sharing distribution could be zero.
Or it could be a large number. The owner gets to decide how much of that large number makes up the profit-sharing pool. Just like owners get to decide how large the manager's "bonus pool" is.
If properly designed, it can also take into account merit-based distributions: if I come up with an idea that saves my company one million dollars a year, then the company can reward my skull sweat by distributing, say, five percent of one years' savings. One company I worked for has a program like that, and it worked wonders for the company's bottom line.
You bring up an interesting questions about union bargaining. I would stipulate that "equitable" is defined that everyone -- union and non-union -- are treated identically for the purpose of earning the extra profit-sharing deduction. So distributions could be divvied based on the employee's other compensation as a percentage of the total non-owner compensation paid at the company. You make one percent of the compensation, you get one percent of the profit-sharing pool. The owner sets the size of the pool based on the company's performance and how much he wants to deduct on the company's tax return.
The tax deduction would be some percentage of the size of the profit-sharing pool; as a starting point, say it's 125 percent. That would be incentive for owners to consider this.