and it seems easier to focus on what other groups pay for those skills, which is not what I want.

The problem is that the skills have different worth and that said worth is different given the skill set. This isn't what you want to hear, but cost to replace is a good metric for this. This includes the quality of the work.

TL;DR Impossible, you can't just split all the money among the employees.

If you want to be fair in this you need to include all the expenses any business has and also reduce that by some multiple, no business can spend all it makes and survive for long.

Businesses have the same risks and problems as people do, with ironically, additional problems and risks brought on by the people themselves, such as embezzlement and theft.

You have to include or calculate for holding back profits to stay in the bank to get the company through a recession, natural disasters, or unforeseen circumstances on the downside, or to buy new production facilities and equipment on the upside.

There is almost always debt service on real estate or existing equipment.

There are lots of costs any business has and must provide for such as defending against frivolous lawsuits, patent trolls, and grifters, as well as the usual ones such as advertising, complying with government regulations, taxes.

For retail and manufacturing, supplies and enough inventory takes up a lot of capital and also financing to make it work.

I think market based mechanisms for calculating pay would be very hard to get away from.

That said, Mondragon (worker cooperative) has a base pay rate and then all positions are assigned a multiplier to determine an individual's compensation.

Just?

Like divide profit by a weighted hours worked average for all employees?

Personally, I'd start at the minimum required to live in the immediate area around the job site and go up from there based on merit/difficulty of the task/importance of task. But then again, I'm not a business person.

There are a bunch of approaches but one I like is to have everyone vote on the relative pay for each role except their own, so customer service doesn't vote on customer service pay ratio but votes on everything else. Once you have agreed upon relative pay you then take the total budget for pay and divide it among the whole staff according to those ratios. Nobody will vote for the CEO to make 300 times what someone else makes but they will vote for higher pay for jobs they don't want to have to hire again for, say shitty jobs or complex jobs. This means the hardest to hire for are retained, the ones who make work easier for others are retained, and the ones who are making life hard for others get reduced. It also means nobody will have to feel that they didn't have a fair shake, they got to vote and voice their opinion but the group has voted. Also, who really feels OK paying someone a pittance? Exactly the type of people who will be pushed out of this type of structure.

Several people doing different tasks

Are you able to expand on the scale and nuance here?

1. Is it in a chain where one persons tasks gives the input to the next persons task? factory line style?
2. How large are the tasks and then roles to fulfill them? IE at a subway (bake bread initially, prepare sandwich on demand) vs a software project (months long work, tasks could potentially months long and need many subtasks)

Some example questions for what I mean

what other groups pay for those skills, which is not what I want.

Why not, and what would be your 'just' criteria instead?

A totally different kind of 'just' approach would be to find out what your workers need for living, and pay them that.

At my company management pay is a fraction of the hourly cost of employees for standard time and materials work.

Add in customer service, sales, and development, and it seems easier to focus on what other groups pay for those skills, which is not what I want.

Why not? Why not value the work based on what others are willing to pay for it?

You can choose whatever process you like. When a market exists then you're still competing with the market. If a market doesn't exist then there's no competition. Either way it doesn't matter what process you choose.