I just harvested some 3787 from a 1.060 dubbel and split the yeast in two jars. One for a 1.080 Trippel this weekend and the other for a 1.090 quad in a couple of months. I would like to use a starter on both to reassure me that the yeast is working. I was thinking about a 2L starter for the Trippel this weekend and a 4L starter for the quad later. What do you think?

If the harvested yeast is less than 4 weeks old, I would direct pitch into the trippel without a starter; plenty of yeast in there to do the job.

I’d make a 1.5L starter for the quad, to revive the yeast and get some reproduction.

Based on these assumptions:

If you had a healthy pitch into the dubbel you can figure between 900B and 1000B cells post fermentation. Split in half, you have 450-500B. Then you can figure a 50% viability after 2 months and come up with 250B. Put that into yeastcalc.com or mrmalty.com and you come up with 1-1.5L starter to get enough cells to pitch.