Speeding up vba code using arrays

Hi there

I have written a macro to do some quite intensive modelling by my standards but it is falling over with the amount of data it needs to work through. It could get up to several 10's of thousands of rows.

I have seen on posts that using arrays can significantly speed things up but I'm struggling to understand how to use them with all the calculations I need to do.
I would be grateful if someone would look at my sample file and give me some advice on how to declare the array/s and then use it/them in the calculations.
The result needs to be captured in column Y.

Please let me know if I need to provide more info.

Many thanks

Never mind. I resolved this myself.