Hi Con, that depends a little on how warm your oven proof setting registers. For many types of ovens, this setting is at 100°F, which is really higher than I like to go with my sourdough starter (or bread dough, for that matter). Ideally, I wouldn't go over 80°F. If you're looking for a warmer spot to stick your sourdough starter, I find boiling some water on the stove and placing it in a Pyrex cup in my microwave (alongside the starter) creates a nice, warm rising environment. If need be, you can refresh the boiling water after an hour or two. 

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