Visa extension and ILR eligibility question

[pintokhan17]

Hi Everyone,
Sorry if this is not the right place to ask the question, but I'm a little confused about what my ILR eligibility dates would be. Just to give you a quick summary before I ask the question:

I was in the UK on a Tier-4 dependent visa in Feb 2018. This was valid till 31-Aug-2019. I made a Tier-2 application on 3-July-2019. I received a visa confirmation letter from the home office stating successful application on 24-Oct-2019 stating I will receive BRP within the next 10 days. However, my BRP arrived in December and had start date of 10-Dec-2019 on it.

With this in view, does my continuous period in-country start from the date on the BRP, or the letter from the home office? Essentially, October or December?

With these dates in mind, I'll be eligible for ILR in 2024. I have to apply for an extension to my current visa, which runs out this year. Time left between expiry and eligibility is approx 2.5 years. Can I apply for and pay for 2.5 years, or is 3 years the only option? Is there even a difference in the amount I will have to pay between 2.5 years and 3?