Naaaaah no need to do a step-up starter. I'm sure there are plenty of viable yeast in your harvest batch that they'll reproduce to proper numbers nice and quick. I don't remember the EXACT number but yeast will always reproduce to their ideal concentration... I believe something like a couple billion cells per milliliter.
Now I'm going to make some assumptions so I can give you a general idea of what you should do.... if any of these are drastically wrong please let me know! I'm assuming:
- You don't have a stir-plate so you'll just be swirling the container every so often to keep yeast suspended and to keep introducing O2 so reproduction goes smoothly.
- You're going to have 5.5 gallons of wort at an OG of 1.075 (just guessing, based on Belgian strong OG's)
- You've harvested roughly the equivalent of one commercial yeast vial/pack
With these assumptions (since you've given me no information
) it looks like you're going to need a ~2.5L starter. If you have a container large enough for that then Mazel Tov! Just make 2.5L of wort at ~1.040 OG and add your yeast. Let it do it's thing for 24-48 hours and let it cold crash (put it in your fridge, covered in a sanitary manner) until you have a nice yeast cake on bottom and a mostly transparent liquid on top. When you're ready to pitch your yeast, decant the top liquid off, but leave a little bit so you have a medium in which to suspend your yeast cake. Swirl that mofo up until you think all of the yeast is in suspension, then pitch into your wort. DONE!
If not (say you have something that can comfortably hold 1.0 - 1.25L) then make a 1L starter, let that work for 24-48 hours, cold crash it 12-24 hours, decant off the liquid (your yeast will make a nice cake at the bottom from cold crashing so you don't need the liquid on top), and add another 1L of wort.... this is the step-up starter you spoke of.