Take the dose you add daily, and use that.

50/300= 20% if you account for the tank, rocks etc.

So what do dose weekly?
About 25ppm NO3.

25/7= 3 ppm or so NO3.

You can dose daily, you can dose 3x a week, or whatever.
The dilutions will occur about the same as a 50-60% if you do 3x 20% a week I'd say.

Just a quick guess.

Why not use a tub for the water or simply add it back slowly so the warmer water and heater can buffer the temps?

Why do all that for a water change?
I add dechlorinator and use the hot water to mix with the cold.

Solves the issue.

Regards,
Tom Barr