There's a yeast pitch calculator link in the yeast section of the recipe editor. It'll tell you what you need and provide basic guidance with slurry. I use that and cross reference it with the Mr. Malty yeast calculator. That one gives more option for closer calculations based on age of the slurry, thickness, etc.
I pitch straight slurry and it tends to have a fair amount of non-yeast stuff and the BF calculator default count of about 1 billion per ml seems to work out about right with fresh stuff. For older stuff, I adjust downward, consulting the Mr. Malty numbers. If you've rinsed the slurry and carefully separated out the yeast layer, you have very dense yeast count. Maybe twice as much as the BF calculator estimates.
A decent rule of thumb is that the yeast from one batch will do at least 2 batches of the same size and gravity. If it's nice and fresh, it might be adequate for 3 but I think that would be pushing it.
If it's a cool-fermented lager, you need a lot more but the yeast calculator will tell you that, too.

 

Assuming your golden strong is a 5 gallon batch at 1.090 starting gravity, a 2.5-3.0 liter starter (1.036 gravity) with 2 tablespoons of yeast slurry will give you a 1.0 pitch.

2 tablespoons of yeast slurry contain about 120 billion cells. Punch that into the calculator and adjust the stater size. I would favor a 3 liter starter to error on the side of caution.

That’s my two cents.

 

Yeah, if you're doing a 1.090 beer, you need a starter.

 

Look at the side of your yeast jar how far far up the jar does the white creamy layer go. How many Mills is the Jar every milliliter of yeast is 1 billion cells so 20 mills 20 billion. This is how i estimate my yeast count.

 

Your yeast is likely to be quite tired after such a big fermentation. Starter, definitely.