Shared Ownership - Staircasing question

Rondoten10

Hello, i wonder if someone more clue up than me could help me with a question surrounding my shared ownership house and purchasing more of it.
So in March 2020 i bought a 50 percent share of a £325,000 house for £162,500.
I am on a fixed rate 2 year deal, after looking at the mortgage statement the outstanding capital in the house when this 2 year fixed term ends will be £150,521.73 so as far as i can see that means that i will have £11,978.27 equity in the house?
If i wanted to staircase up to 75 percent (buying an additional 25%) Assuming the house is the same price.

75 percent of £325,000 is £243,750.00
So 10 % deposit would be £24,375.00
Minus equity of £11,978.27
Would mean i would need an additional deposit of £12,396.73
Do my figures check out?

My next question is what if the house value has gone up or down, what does that affect?

Thanks in advance