Changing Refrigerant in Central Air from r22 to M099

My plumber has been trying to sell me a whole new HVACstating that r22 has become to expensive and hard to get. But after a little research I found a supposed replacement refrigerant

DuPont

Isceon MO99

Looks like 25 lbs will run about $220.00 + $39.00 shipping on Amazon.

Then all you do is recover the r22 from the current system, check the seals and replace the gas. The law says this must be done by a licensed professional and that is fine. He then has a lot of r22 he can sell to someone else.

I also know that if I know what the problem is and the solution, I protect myself from service professionals that may want to sell you something you don’t really need.

My immediate question is how do I find out or calculate just how much gas my system needs?

@ladytech @oldturkey03 do you know anything about this. It could be a very important answer for lots of homes and a whole lot of $?