Tile grout in small quantity

Hi,
Replacing a couple of tiles over the kitchen countertop. Need just a bit of grout, but instructions on the box are for big quantity.
What's the rule of thumb for mixing grout powder with water in small amounts? I know you have to wait 10 min before using it, after it's mixed.
Is the ratio of grout to water that critical, or should I just go by what seems right?
Regards,
RichK
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I have heard that the consistency of toothpaste is about right, and that seems like a pretty good guideline to me.
JK
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