best glycol and mix for my boiler system?

A few suggestions:

You do not want to blend different brands as they use different inhibitors that may not get along.

My process would be to squirt in a cleaner and run that for a day to get the system to a good starting point for new glycol. Flush it out well.

This system cleaner comes in a squirt can, screw it onto a purge or drain valve, hose thread, open valve, and pull trigger. Be sure it is tightly connected as the can empties in seconds!

Choose a glycol brand and the %. Most brands have info on the container for protection. Generally 30- 35% is adequate, but decide on the lowest temperature you want protection to.

This Fernox Alphi 11 at 35% will flow down to, -11°F burst at -40°F

You will need some way to pump it into the system and purge air out. A 5 gallon bucket or plastic garbage can allows you to pump out, and purge back int the bucket until you do not see or hear air bubbles. This can take some time depending on the piping, zones and amount of fluid.

Simple and inexpensive is a 5 gallon bucket with a small submersible type pump from the Home Stores, 1/2hp is plenty.

Or find pool or spa pump on Marketplace and build a system like this.

Some hydronic wholesalers have pump carts to rent or loan, that makes it a simple, safe job.

I would buy pre-mixed glycol and use it right out of the bucket. Then you have the correct blend.

You do not want a fill system connected that could add water and dilute the mix, as you have learned.

Some choose to add a glycol fill tank to add fluid as air bleeds out.

Glycol is very slow to give up entrained air, it may take days for a good air purger to get the system 100% air free. You will need to boost pressure up to 12 psi a few times as air slowly comes

@clydesdale in simple terms, antifreeze reduces the heat output. For example, you may get away with 150 degree water and heat your home on the coldest night. Add thirty percent glycol mix and you might need 170 degrees.. Add fifty percent glycol mix and you might need 190 degrees.

In addition the fluid does not flow as well when antifreeze is added. You may need a slightly larger circulator.

For these reasons and cost you don't want to use more glycol/antifreeze than you actually need. One method to determine the capacity of a relatively small system is to fill the system with water and vent all of the air out. Then drain the system into empty five gallon buckets.

you could drain the part of the system prone to freeze. You would need two valves to isolate that section then repressurize the balance of the system. to keep it going

Any brand of cleaner will work, Fernox, Rhomar, Axiom, sentinel

If you don’t use an aerosol can you will need to pump it in.

The nice people here that do this for a living are suggesting 30-35% why would you want to go 50%?

Just get a 5 gal bucket straight glycol.

Something that is almost always overlooked when discussing Cryo-Tek 100 is the fact that it's only 55% concentration straight from the bucket. Cutting it 50/50 would put you at 27.5%, which is borderline (25%) grounds for bacterial growth. 30-35% is typical with 30% as an absolute minimum IMO, so you'd need to calculate your total system volume and then buy enough concentrate to premix that amount to your desired percentage, and then pump it in. Get yourself a refractometer to check when you're done, and you may need to make adjustments by draining some out and refilling with more concentrate as you'll never get all of the existing fluid out so it'll dilute your new stuff. 2 parts Cryo to 1 part water is roughly 36.5% so that's easy to premix and would likely get you pretty close.