It's not a good idea to make a starter with dextrose. The yeast will "get used to" chewing on the extremely simple sugar you've provided and then turn their noses up at the tasty tasty maltose in your beer***. This can cause them to stall out and not finish fermenting your beer. You should try to use malt extract if you can, or you can make a small batch of unhopped beer with 2-row. In either case you should shoot for a gravity of around 1.040.
***What's really happening (if you care: bio wonk stuff coming) is that as the yeast are propogating during the lag phase, they're switching all of their metabolic machinery to eat dextrose- switching on genes that produce dextrose-processing enzymes and turning off genes that make enzymes to work on other sugars to save energy (anaerobic fermentation is not an efficient way of creating energy for yeast). Once they've stopped reproducing (starter is done) it's very difficult for them to switch back, as it's fairly energy intensive (remember, anaerobic conditions are energy-scarce) and they already have the dextrose machinery in place.
I can't be arsed to keep up this list of what's in the fermenters, but hey, check out the cool brewery I own!
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