Infidel

There really does seem to be a "break point" as some folks here call it.
With your super high CYA your break point will be way up there. Sometimes the advice given here is a bit conservative because some folks with vinyl lined pools might take the method and end up bleaching their liner. Not your problem with a concrete/plaster pool.

Vac to waste will be a good idea. Shut down for a couple hours, let it settle, set up the vac, switch to waste then start the pump and it will be most effective.

When you finish that backwash the filter, get a small bag of Diatomacious Earth (DE). Set the valve on filter and note carefully the pressure reading. Then add DE to the skimmer a cup or two at a time until you see the pressure go up 1 PSI. Keep t

he pump running like that for a couple days. The DE will help quite a bit to get the finest dead algae out. Don't inhale. Not the grass... the DE. The pool grade DE is not good in your lungs.

You should by now have seen the famous "best guess" chart for the chlorination level vs CYA levels. Take good note. You will need to keep the chlorine at pretty high levels until you get the water exchange done. Try diluting the CYA test with distilled water to see just how much above 100 you are. Keep your PH in the 7.2 - 7.3 range and the CL you do have will be more effective.

Good Work.