SPIT MK I/II and over boost

[Crumpp]

I did, it still says constant IAS to FTH and linear reduction above optimum climb.

It is a pretty simple concept to grasp. Climb speed changes with altitude.

That is it and all you need to know!!

If it does not change with altitude or is a simple linear then for practical pilotage it is simpified to get the pilot in the ballpark!

Again, very simple concept that does not require in depth discussion.