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A mug of boiling water sitting in the oven will do much the same. That's quite a low seed percentage too.
My kitchen is sitting around 17-18deg I guess, and I feed once a day with 20g starter, 40g water and 20g each white and wholemeal, with cool-lukewarm water. I could probably feed twice a day if I wanted to keep it really young but it seems fine as it is and I tend to feed once the night before and then make the levain in the morning so it gets fairly lively
At the moment when feeding it's 15g starter, 100g water (around 25-27c) and 100g strong bread flour. Can't do the oven thing as our light is bloody bust. Looked at a proofing box, seems a tad too much right now...