Not sure of how to calibrate fuel gauge after battery replacement

Hello all, I hope you are well.

I recently replaced my Oneplus 8 battery using an iFixit battery and it was a very successful replacement.

I attempted the calibration using the guide from iFixit and many various sources to calibrate the fuel gauge. That first time, I noticed that the battery lasted a long time in general, and lasted a while at 1%. It died, I plugged it in and when it reached 1% again I turned it on and left it to charge.

This worked well for 1 day - but after a few days, the battery percentage would drop quite quickly to 1% and then last 2 to 3 more hours on 1%. Since then, I have attempted the calibration process a few times but I can't quite get the calibration to stick.

• Is there anything else I can try or am I missing anything in particular?
• Could there be any fault with the battery? If so, can I diagnose it in any way?

Any guidance would be much appreciated. I was so hopeful to bring my phone back to life from its poor battery life, but I'm sad to see that it's even worse now :(