One way to get started is to imagine what would happen if you planted 1 hectare with Sitka Spruce.
This would cost £120 000.

After 10 years, you would get £10 000 from the 10-year thinning:
-120 000 + 10 000 = -110 000 so you have a loss of £110 000 after 10 years.

After another 10 years, you get £40 000 from the 20-year thinning:
-110 000 + 40 000 = -70 000 so your loss after 20 years is £70 000.

You could then choose to chop down your forest and sell the wood after another 10 years. This would bring in £358 000.
-70 000 + 358 000 = 288 000. So after 30 years, you have a profit of £288 000.

However, you could choose instead to keep the trees and cut down the forest after 40 years. This would bring in £513 000.
-70 000 + 513 000 = 443 000. So after 40 years, your profit would be £443 000.

Perhaps you could set up a spreadsheet to calculate the costs and profits for each type of tree!

If you want to maximise profits after 100 years, you could investigate planting a forest for 50 years, and then cutting it down and planting another 50-year forest.
Or planting a 60-year forest and a 40-year forest.
Or two 30-year forests and a 40-year forest.
Or...