Yeast Starter Size And Correct Procedure Wanted Please

Made now approximately ten brews now, i am only doing extract brewing at the present but trying to improve things the best that i can, my last brew a Bulldog London Porter i made a yeast starter 900ml from a yeast slant, it obviously went ok as its fermenting furiously in my bucket and was doing so within seven hours i even had to make a blow off tube for the first time.
I am after clarifying the procedure, i made a 900ml starter spun on my stir plate for twenty four hours then chilled , decanted and readded a new dme mix back up to 900ml.
Reading other posts i should have made a smaller starter to start off with and then built up to the final amount, is that correct?
It thats the case, how much is best to start of with it?
How much do i increase each time , ie do i double up each time or less or more?
How long should i spin each for?
Do i need to chill and decant wort off each time and add the increased volume of wort?
Thanks all.

The typical step up quantities are something like 800, 1200, 2000. Those are not set in stone.
Yes, chill and decant each step.
Personally, I use 1.040 wort saved from previous brews, boiled and canned in 1 qt. canning jars.
I'll pull out 1 qt., and use it for a first step.
Then chill and decant.
Then step up with 2 qts.

If I have started with a fresh yeast pack, the 2 steps is enough to decant and save the slurry 3 ways in pre-boiled water (which will be sterile and void of oxygen).
The yeast/sterile water slurry will last a long time at 34F.
With those three yeast jars, I'll use one to step up and pitch in my next brew.
I'll save the second one for later.
I'll double step up the third and start all over again.

Make sense?

Yes thankyou very much indeed