Stepping up in a 2 stage yeast starter

I have to step up to a second stage for a yeast starter today, and can't find information on how to properly determine OG for the second stage. I did a 2L @ 1.040 last night, and will be transferring that batch to a 3.5L starter tonight. Ideally, I'd cold crash and decant between stages, but I need to get the second stage going tonight so I can properly crash before brew day...

Since the 1.5L batch will be partially fermented, is there any rule of thumb for the OG for the second stage to get the combined starter-wort to 1.040? Yes, I realize I could take an OG reading, but on such a small batch the wort needed for the sample seems to be a pretty hefty fraction. Should I just prepare another 1.5L at 1.060, assuming that the 2L has fermented down to about 1.018-1.020 and that they'll combine to be ~1.040? Or is it necessary to take a sample?

I'd just prepare more 1.040 wort and add it. If the average gravity of the wort is a tad below 1.040 it's not going to inhibit yeast growth.

Ah... Very helpful, thanks! I had been operating under the assumption that the doubling rate was tied in part to the gravity, but I'm now seeing a few notes online that suggest its more dependent on volume (provided there is *some* sugar involved for nutrients). Awesome.